The sum of two different prime numbers is 10 . The product of these two numbers is
(A) 24
(B) 21
(C) 16
(D) 9
(E) 7
The prime numbers smaller than 10 are $2,3,5$, and 7 .

The two of these numbers which are different and add to 10 are 3 and 7 .

The product of 3 and 7 is $3 \times 7=21$.

ANsWER: (B)