septwibowo wrote:
Dear expert, I chose C here, but I still need a valid reason why we must write "have" or helping verb in the right hand? Why "but also" alone is not enough?
Thanks!
Two problems with "but also":
1. The complementary part "not only" is missing. However even if "not only" were there, there could be parallelism issue:
a. From 1965 on, NOT ONLY the region’s standard of living has soared, but also unemployment and prices.: Wrong - parallelism between clause (the region’s standard of living has soared) and noun phrase (unemployment and prices). Note that "have soared" in the second element of the parallel structure cannot be omitted because the word "have" does not occur in the first part of the sentence.
b. From 1965 on, NOT ONLY the region’s standard of living has soared, but also have (soared) unemployment and prices.: Correct - compares clause with clause. The word "soared" can be eliminated from the second element because the word already occurs in the first element of the parallel structure.
2. Now ignore the above error and consider that "but also" can be used without "not only" (in fact it can be, but not ideally in GMAT). Now (in option A) arises a parallelism issue ( similar to the one mentioned in 1a above.) The parallelism is between clause (the region’s standard of living has soared) and noun phrase (unemployment and prices). The part "have soared" in the second element of the parallel structure cannot be omitted because the word "have" does not occur in the first part of the sentence.
Thus option A ("but also" alone) is wrong in 2 ways.