What is the greatest prime factor of 8^13 - 2^30?  Choose the correct option.
A) 23 B) 37 C) 59 D) 73 E) 107
8^13 - 2^30   =(2^3)^13 - 2^30   = 2^39 - 2^30   = 2^30 (2^9 -1)   = 2^30 * (512-1)   = 2^30 * 511   = 2^30 * 7 * 73   The greatest prime factor is 73.   The answer is 73.
<result>D</result>
What is the greatest prime factor of 8^13 - 2^30?