4. Which of the following is not the sum of two primes?
A 5
B 7
C 9
D 11
E 13
4. D All primes except 2 are odd. So the sum of a pair of primes cannot be odd, and so cannot be prime, unless one of the pair is 2 . We note that $5=2+3 ; 7=2+5$; $9=2+7 ; 13=2+11$. However, $11=2+9$ and so it is not the sum of two primes, as 9 is not prime.