I fell for the trap.C is correct.
S is a set of integers (not necessarily just positive integers).Question is "Is every multiple of 5 is in S?".
#1 misses negtive multiples (-5,-10...)
#2 misses positive multiples (5,10,...)
So,together covers all multiples.
That is the beauty of the DS questions.Small twist to a relatively easy question can differrentiate best people from the good.
Actually, I'm not sure negatives are considrered as multiples. I thought that (2) brought that info but then I changed my mind and thought that (2) actually brings 0 which (1) does not.
Also, when reading the question again my heart is going to D (even if i know it's not the answer).
(1) No --> sufficient
(2) No --> sufficient.