Definition: You are given a math word problem and you are supposed to make use of mathematical concepts like probability, combinatorics, linear equations, sequences/series, or simple operators like addition, subtraction, division, or multiplication to answer the given question. Choose the best option from the 5 given options and report that option.
Input: Question: If a and b are prime numbers, which of the following CANNOT be the sum of a and b?

 Option A: 5

 Option B: 9

 Option C: 23

 Option D: 16

 Option E: 13

Option C