What is the greatest prime factor of 4^18 - 2^30?  Select the correct option
A) 2
B) 3
C) 5
D) 7
E) 11
4^18 - 2^30   =(2^2)^18 - 2^30   = 2^36 - 2^30   = 2^30 (2^6 -1)   = 2^30 * (64-1)   = 2^30 * 63   = 2^30 * 7 * 9   = 2^30 * 3^2 * 7   The greatest prime factor is 7.   The answer is 7.
<result>D</result>
What is the greatest prime factor of 4^18 - 2^30?