Which of the following is a prime number?
(A) 20
(B) 21
(C) 23
(D) 25
(E) 27
20 is not a prime number, since it is divisible by 2 .

21 is not a prime number, since it is divisible by 3 .

25 is not a prime number, since it is divisible by 5 .

27 is not a prime number, since it is divisible by 3 .

23 is a prime number, since its only positive divisors are 1 and 23 .

ANSWER: (C)