Hey ardz24,

Assuming we've already eliminated D and E for being fragments, let's look more closely and A, B, and C.

A key issue here is the change between the plural "suggest" and the singular "suggests" - this means that we probably an agreement error. We need to figure out what the subject of the verb "suggest"/"suggests" is to determine which one we want. Looking at the sentence and cutting out all of our extra descriptors, we see that the sentence says

"A surge ... and a drop ... suggest ..."

We have two things that are suggesting - a "surge" and a "drop" - so we have a plural subject. This means we want a plural verb to match. So we can eliminate B for using a singular verb.

To decide between A and C, we should notice that C introduces "as have been previously thought". Again finding the subject:

"... the economy might not be so weak as [the economy] have been previously thought"

So we have a singular subject ("economy") and a plural verb ("have"), which is incorrect. We can eliminate C, giving A as the correct answer.

Quick note: in C, we also have passive voice (the economy was thought about by analysts) instead of active voice (analysts thought about the economy). In general, active voice is preferred to passive voice. However, passive voice is not always wrong on the GMAT, so this is only a secondary reason to eliminate. An answer choice that is grammatically correct but uses passive voice will always be better than an answer choice that is grammatically incorrect but doesn't use passive voice.