Which number should replace both the asterisks in (* /21) x (*/84) = 1 ? Choices:  A) 21  B) 42  C) 63  D) 72  E) 168

Rationale: "let ( y / 21 ) x ( y / 84 ) = 1 y ^ 2 = 21 x 84 = 21 x 21 x 4 y = ( 21 x 2 ) = 42 the answer is b ."
Answer: B