Comma usage

[duyng9989]

Aerugo, also known as verdigris, is the green "bloom" visible on many copper items, and is produced, like iron rust, over the course of time by the exposure of the metal to the oxygen in the atmosphere.

a.visible on many copper items, and is produced
b.that is visible on many copper items, and which produces
c.visible on many copper items, and produces
d.that is visible on many copper items, and that produces
e.which is visible on many copper items, and which is produced
Source: MAnhattanGmat prep
I have a question: The answer is A. I picked it correctly but am very skeptical. Why the comma before "and is produced"?

Simplyfied version:
STH is the green boom, and is produced?

I thought that there would be no comma before "and" separating two items?

[Willy]

Aerugo, also known as verdigris, is the green "bloom" visible on many copper items, and is produced, like iron rust, over the course of time by the exposure of the metal to the oxygen in the atmosphere.

a.visible on many copper items, and is produced
b.that is visible on many copper items, and which produces
c.visible on many copper items, and produces
d.that is visible on many copper items, and that produces
e.which is visible on many copper items, and which is produced
Source: MAnhattanGmat prep
I have a question: The answer is A. I picked it correctly but am very skeptical. Why the comma before "and is produced"?

Simplyfied version:
STH is the green boom, and is produced?

I thought that there would be no comma before "and" separating two items?

Wow! there are so many commas in this question but I think all are placed correctly. Try to remove the comma before AND and sentence will sound as if 'Aerugo' is producing iron rust and this would not make any sense.

I think as written now 'iron rust' is used as example of 'copper items'.

Also, if you try to remove the comma before AND you may need to remove the comma after Produced and whole meaning and grammatical sense will change.

So, I believe as written sentence is best!

[jlucero]

First off, let's put questions about GMAT Prep questions in the proper forum. I'll move this thread over there for future students.

Second, if it's in the correct answer (or in this case, all of them), learn to accept the GMAT's answer as correct.

Third, the GMAT doesn't test rules of comma usage, so on test day, it's not worth your time to investigate.

Finally, to actually answer your question as to why it's acceptable, it's because it makes the sentence cleaner and less ambiguous. Let me throw out the two modifiers and rephrase the sentence as:

Aerugo is the green "bloom" visible on many copper items, and is produced over the course of time by the exposure of the metal to the oxygen in the atmosphere.

Notice the parallelism here involves Aerugo doing two things: Aerugo is X (the green bloom) and Y (produced over time). But the problem is that there is a word WITHIN X that could also be parallel to "produced"... "visible". So if we're not careful with this parallelism, we could get:

Aerugo is the green bloom: visible on copper items and produced over the course of time"

Now we could debate about how different this meaning would be from the original, but the point is that the GMAT (or whoever wrote this sentence) did not intend this meaning, so they clarified this twofold: (1) adding a second "is" to show exactly where parallelism would be found and (2) putting a comma to offset the two clauses. This is 100% acceptable, and illustrates how the GMAT prefers clarity of meaning over rules of commas (which differ by style guide used).