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: "given : n = 20 ! n = 20 ! . the highest power k for which 2 ^ k is a factor of n can be found with the above formula : k = 20 / 2 + 20 / 4 + 20 / 8 + 20 / 16 = 10 + 5 + 2 + 1 = 18 answer : d ."
Answer: D