What is the greatest prime factor of 2^8 - 1?  Choices:
A) 11  B) 13  C) 17  D) 19  E) 23
2^8-1 = (2^4-1)(2^4+1) = 15*17   The answer is 17.
<result>C</result>
What is the greatest prime factor of 2^8 - 1?