The sum of two different prime numbers is 10 . The product of these two numbers is
(A) 24
(B) 16
(C) 4
(D) 21
(E) 9
The prime numbers that are less than 10 are $2,3,5$, and 7 .

Thus, the only two different prime numbers whose sum is 10 are 3 and 7 .

The product of these two numbers is $3 \times 7=21$.

ANSWER: (D)