21 = 3*7 and 3 and 7 are both prime.
22 = 2*11 and 2 and 11 are both prime.
23 = You might think of 1*23, but 1 is not prime.
24 = 3*8 or 4*6, or 2*12, or 1*24, but none of
those is the product of 2 primes.
25 = 5*5, 5 is prime but the two 5's are not different numbers.
26 = 2*13 and 2 and 13 are both primes.
27 = 3*9 but 9 is not prime.
28 = 4*7 or 2*14 but neither is a product of two primes.
29 = You might think of 1*29, but 1 is not prime.
So only 21, 22, 26 can be written as the product of two primes.
Therefore the answer is (D) Three
Edwin
