If n is the product of integers from 1 to 20 inclusive what is the greatest integer k for which 2^k is a factor of n?
.  Choices:	A) 10	B) 12	C) 15	D) 18	E) 20

Rationale: 20 ! = 1 * 2 * 3 * 4 * 5 . . . * 19 * 20 ( this is 20 factorial written as 20 ! ) n = 1 * 2 * 3 * 4 * 5 * 6 * 7 . . . . . * 19 * 20 how many 2 s are there in n ? one 2 from 2 two 2 s from 4 one two from 6 three 2 s from 8 and so on . . . when you count them all , you get 18 . answer : d
Answer: D