For a positive integer n, if 5^n is a factor of 20!, but 5^n+1 is not a factor of 20!, what is the value of n?  Choose the correct choice from the following answers.
A) 4
B) 6
C) 7
D) 8
E) 9

Rationale: 20 ! has four 5 ' s 20 / 5 = 4 thus 5 ^ 4 can completely divide 20 ! and 5 ^ 5 can not divide 20 ! so , answer will be ( a ) 4
Answer: A