If a and b are prime numbers, which of the following CANNOT be the sum of a and b? Options
A) 5 B) 9 C) 23 D) 16 E) 13
All answer choices but C are odd. In order odd number to be the sum of two prime numbers one must be even, so 2, and another odd.   Now, 23=2+21. Since 21 is not a prime then 23 cannot be written as the sum of two prime numbers.   Answer: 23
<result>C</result>
If a and b are prime numbers, which of the following CANNOT be the sum of a and b?