16. Which of the following values of $n$ is a counterexample to the statement, 'If $n$ is a prime number, then exactly one of $n-2$ and $n+2$ is prime'?
A 11
B 19
C 21
D 29
E 37
16. E Since 21 is not prime, it cannot give a contradiction to the statement because $n$ must be prime.
The primes 11, 19, 29 don't give a contradiction because exactly one of $n-2$, $n+2$ is a prime for each of them: 11 ( 9 is not prime, 13 is prime), 19 (17 is prime, 21 is not prime), 29 ( 27 is not prime, 31 is prime).
However for 37 , which is prime, this does give a contradiction.