Strengthen arg by consolidating the argument feasibility?

[RaffaeleM39]

I too selected C.

After reading the answer key and this post I still have questions

The prompt says:

much of the waterfront in these cities was never developed aesthetically and instead was left to industry and commerce.

So some (much) of the waterfront was left to industry and commerce
The conclusion is:

A developer who wishes to make a large profit would be wise to buy urban waterfront lots and erect residential buildings on them.

The first question is:
1. were does "urban" come from? What's the difference between "urban" waterfront and the waterfronts referred to previously in the passage?

The correct answer says:

B. Homeowners will be willing to spend large sums on residential properties in traditionally industrial or commercial districts.

And the explanation says:

(B) CORRECT. This choice states that homeowners will be willing to spend large sums of money on residential properties in traditionally industrial or commercial districts. Since we know from the argument that urban waterfronts have traditionally been industrial, this fact strengthens the claim that a developer can make a profit on urban waterfront properties.

What?
Answer B never mentions waterfront properties or the like. It only mentions "residential properties in traditionally industrial or commercial districts".
Are ALL "residential properties in traditionally industrial or commercial districts" waterfront properties? No. Some "residential properties..." are not waterfront properties.
Answer B does not make a distinction between waterfront and non-waterfront "residential properties....". Perhaps, "Homeowners will be willing to spend large sums on residential properties" only if these are non-waterfront properties. Then the argument would be weakened.

Stated otherwise, the question tells us that "much of the waterfront in these cities [...] was left to industry and commerce" but nobody ever told us that industry and commerce took care only of waterfront properties and nothing else (and not, for example, properties in the city center).

Answer B uses "industrial and commercial districts" as a synonym for "waterfront properties taken care by industry and commerce", and I have a problem with that!

My second question is:
2. What is wrong with my reasoning?

For me Answer B was simply a focus shift (thus a wrong answer)

[Sage Pearce-Higgins]

Where does "urban" come from? What's the difference between "urban" waterfront and the waterfronts referred to previously in the passage?

The passage starts off by talking about waterfront properties in 'American coastal cities'. The word 'urban' in the conclusion refers back to this. Importantly, however, the argument states that waterfront properties are desirable in general. That, for me, is a big clue. When reading this I think 'Aha, but is there some reason why the waterfront properties in cities might not be desirable?'.

Answer B never mentions waterfront properties or the like. It only mentions "residential properties in traditionally industrial or commercial districts".
Are ALL "residential properties in traditionally industrial or commercial districts" waterfront properties? No. Some "residential properties..." are not waterfront properties.

Of course. The answer doesn't have to mention waterfront properties. Since we already know that 'much of the waterfront in these cities...was left to industry and commerce' we can combine this with the mention of 'traditionally industrial and commercial districts' in answer B. Sure, there are industrial and commercial districts elsewhere just as there are residential districts elsewhere, but that's not what the conclusion is concerned with.

Answer B does not make a distinction between waterfront and non-waterfront "residential properties....". Perhaps, "Homeowners will be willing to spend large sums on residential properties" only if these are non-waterfront properties. Then the argument would be weakened.

Sure, and perhaps the developer does a bad job, and perhaps there's an earthquake... Don't build your reasoning on 'perhaps'. Actually, we're told in the argument that 'today, however, waterfront properties are generally seen as prestigious', i.e. that they are desirable.

Answer B uses "industrial and commercial districts" as a synonym for "waterfront properties taken care by industry and commerce", and I have a problem with that!

Where is the phrase 'taken care' used? The two phrases we need to connect are 'waterfront in these cities was...left to industry and commerce' and 'traditionally industrial or commercial districts'. I can share some of your frustration, as words are inevitably somewhat imprecise, but these phrases are just the kind of equivalence that you'll meet on the GMAT.

What is wrong with my logic?
(I suspect that the word "Many" in Choice C makes it wrong?)

Hopefully I've shown the connection that you need to make. As for answer C, in order to make a large profit, a developer doesn't need 'many' lots. Actually, if there are too many lots available, then it's likely that other developers will get involved and drive the price down. In any case, you need to find the answer that 'most strengthens' the argument.
I would recommend that you do the Problem Set at the end of Chapter 5 of the CR strategy guide.

[SahilM634]

I have a more fundamental question here. The question stem states: "Which of the following, if true, most strongly supports the claim made about urban waterfront properties?". What is this claim about urban waterfront properties. I understand that the conclusion of the passage is that developers who wish to earn profits should purchase such properties and erect residential buildings there and that option B completes the argument. However is this a claim about "urban waterfront properties"?

[Sage Pearce-Higgins]

Critical Reasoning (and RC) problems often expect you to "match up" terms from questions or answer choices, and the passage given. Here, the phrase 'urban waterfront' is used only once in the text, in the sentence: "A developer who wishes to make a large profit would be wise to buy urban waterfront lots and erect residential buildings on them." The bold part here indicates that the author is making a claim. I believe that this is the only claim made in the passage (the rest is presented as mere information). Remember that the conclusion of an argument is the same as a claim. When I make an argument, I have a claim (such as 'I think it'll rain tomorrow') supported by some evidence (such as 'It's getting cloudy.').