Since oil was discovered within the city, the standard of living in Dubai has soared,but the cost of living too.
A.but the cost of living too - what needs to follow ,but (,FANBOYS) is an independent clause. Notice the
verb is missing. Going by the sound- The sentence abruptly ends with too, as if the sentence is left incomplete.
B.
and also the cost of living - on the GMAT you will see options that have and also, which is awfully awkward/redundant.
C.but so has the cost of living- Correct.
D.
and so also the cost of living- same error as B
E.but so
did the cost of living - the auxiliary verb did is not correct.
Consider the below example: The iPhones have been designed to improve user experience, and they do.
The sentence is incorrect because of the presence of incorrect verb "do". The correct sentence will use "are" instead of "do".