Question

Which is a prime number?

A.
21

B.
31

C.
57

D.
91

Answer 1

31.

Answer 2

Because the prime factors of 21 are 1, 3, 7; the prime factors of 57 are 1, 3 and 19; the prime factors of 91 are 1, 7, 13 and the prime factors of 31 are only 1 and 31.

Answer 3

31