Hi ziyuelau,

This DS question is essentially just a 'ratio' question, although you can approach the math in a couple of different ways.

We're told that Set B is a SUBSET of Set A (meaning that every element in Set B is also in Set A). We're asked for the number of elements in Set A.

1) Set B has 14 elements

This Fact tells us nothing about the number of elements in Set A. Since Set B is a subset of Set A, we know that all 14 of those elements are in Set A... but there could be additional elements that are unique to Set A. Thus, Set A contains AT LEAST 14 elements (re: 14, 15, 16, 17, etc.), but we don't know exactly how many.
Fact 1 is INSUFFICIENT

2) Exactly 80% of the elements in set A are NOT in Set B

This Fact tells us the ratio of shared elements (20% = 1/5 of the elements in Set A are also in Set B), but we don't know the exact number of elements in Set A (that total would have to be a multiple of 5 though).
Fact 2 is INSUFFICIENT

Combined, we know...
-Set B has 14 elements
-Those 14 elements represent 20% of the elements in Set A.

Since those 14 elements are 1/5 of the total elements in Set A, Set A must have (14)(5) = 70 elements in it.
Combined, SUFFICIENT

Final Answer: C

GMAT assassins aren't born, they're made,
Rich