Unconvincing CR question

[duyng9989]

Hi.

I have a question, this is from GMAT prep software:

According to the Tristate Transportation Authority, making certain improvements to the main commuter rail line would increase ridership dramatically. The authority plans to finance these improvements over the course of five years by raising automobile tolls on the two highway bridges along the route the raile line serves. Although the proposed improvements are indeed needed, the authority's plan for securing the necessary funds should be rejected because it would unfairly force drivers to absorb the entire cost of something from which they receive no benefit.

Which of the follwoing, if true would cast the most doubt on the effectiveness of the authority's plan to finance the proposed improvements by increasing bridge tolls?

C: Between the time a proposed toll increase is announced and the time the increase is actually put into effect, many commuters buy more tokens than usual to postpone the effects of the increase.
D. When tolls were last increased on the two bridges in question, almost 20 percent of the regular commuter traffic switched to a slightly longer alternative route that has since been improved.

The answer is D.

I know that ROn does not want to challenge the the correct answer but I found the reasoning of the correct answer unconvincing.

GMAT said that:

"what casts doubt on how well the financing plan would work? Any financing plan is based on estimates of costs and revenues, and any factor that significantly increases costs or lowers revenues threatens the defectiveness of the plan. The authority's plan makes a revenue projection based on the current number of drivers who use the bridges and thus will pay the increased tolls. If there is a precedent that significant percentage of regular commuters had previously used an alternate route in order to avoid the increased tolls on these specific bridges, the the revenue basis for the financing plan is considerably undermined. If hat substitute route has since become an even more appealing alternative, the effectiveness of the plan is further threatened.

C: Revenue lost to token hoarding is INSIGNIFICANT compared to the revenue gained from five years of increased tools
D. Correct.

What I am unconvinced is:
1. The premises do not say anything about how the authority's plan to make revenue projection. How do we know the authority's plan based on the current number of drivers who use the brdiges and thus will pay the increased tolls?
2. Choice C said that: the token hoarding is INSIGNIFICANT => how do we know it is insignificant? If there were enough people buying A LOT of token, that would be a huge amount. If an individual buy a large quantity of token enough, he or she can delay the increase for long time.
3. I stayed away from choice D because it specifically mention 20% of regular commuter traffic switched. How do we know 20% is a large number???

Could you please provide an alternative explanation? GMAC is truly a monopoly in the way it dictates what is true what is wrong.

Thank you.

[RonPurewal]

* as for your question about "how do we know the relative magnitude of 20%?" ...
good question!
the answer: you don't. there are no grounds for interpretation of that percentage, since neither (a) the exact amount of token-hoarding and (b) the planned % increase in fares is described in the passage.

but, see, this is good news. when you don't know about a factor, then the problem becomes simpler, not more complicated -- because you can just ignore that factor, or treat it as part of an "all else equal" / ceteris paribus type assumption. (there's nothing else you can reasonably do in such a case; it's certainly not ok to just assume that it goes one way or the other.)

--

because we don't know the relative amounts, we should just ignore them. that leaves only one substantive difference:

* the token-hoarding is a temporary effect. even if someone hoards three years' worth of tokens... after three years, guess what.

* switching to an alternate route, on the other hand, is an effect that can persist indefinitely. i.e., commuters who've made the switch can keep using the alternate route forever and ever, should the circumstances so dictate.

there you go.