If c and d are prime numbers, which of the following CANNOT be the sum of c and d?
Select.
A) 5  B) 9  C) 13  D) 23  E) 33
All answer choices but D 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>D</result>
If c and d are prime numbers, which of the following CANNOT be the sum of c and d?