[problem deleted because source was not cited.]
But the solution is given as A, because in statement B what if a=b. Then the greatest common factor will be a (or b since a=b). Hence this statement is insufficient, because
CASE 1 - IF a b are prime and a <> b, then GCF = 1
CASE 2 - IF a b are prime and a = b, then GCF = a
There statement 2 is insufficient.
Now my question is if this were an actual GMAT question, do we have to consider the possibility that a = b? Since the question has used 2 separate alphabets a and b to denote 2 positive integers, why do we have to conider the possibility that a = b?