If a and b are prime numbers, which of the following CANNOT be (x + y)?  Pick one.
A) 5 B) 9 C) 13 D) 16 E) 23
All answer choices but E 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>E</result>
If a and b are prime numbers, which of the following CANNOT be (x + y)?