A - John is bigger than Joe is - the version that includes a conjugated verb is preferred in the GMAT

B - 1) This year's GDP is higher than last year - INCORRECT - you are comparing GDP and last year
2) This year's GDP is higher than last year's - CORRECT
3) This year's GDP is higher than last year was - INCORRECT - no subject in the clause after "than" OR if "last year" is the subject, then you are repeating the mistake from sentence 1
4) This year's GDP is higher than last year's was - CORRECT

C)
1) Like John, Linda likes flowers - CORRECT
2) Like John does, Linda likes flowers - INCORRECT, "like" is used to compare nouns and there should be no verb after "like"/"unlike"

Hi Kaisa,

Could you please help me understand the question A.

(A)
1) John is bigger than Joe
2) John is bigger than Joe is (my guess)

My guess is that in (1)"is" is omitted and hence is more concise.

It is true that the first sentence might seem better because it is more concise but it is actually less preferred in the GMAT than the 2nd sentence. The use of the verb "is" makes the comparative structure "fuller" or more complete.

One more Is it grammatically correct to say "GDP is higher this year than last" because we compare this year (object) vs. last (object)? Or would it have to be "GDP is higher this year than last's"?