If j is a multiple of 12, and k is a multiple of 21, then jk must be a multiple of which of the following?

One way to approach this problem is to find the least common multiple of both integers:

12: (3)(2)(2)
21: (7)(3)

Least common multiple is therefore (3)^1 * (7)^1 * (2)^2 = 84.
Of the answer choices presented, 28 is the only divisor of 84.

Correct answer is D

Assume J = 24 and k =42 and jk = 24 X 42 = 1008

1008/8 = 126
1008/28 = 36

Accordingly, both choices A & D should be correct, not only D.

Thank you.

Hawer