GMAT Problem Solving (PS) | Re: If a and b are prime numbers, which of the following CANNOT be the val

Let’s evaluate each answer choice:

A) 9

Since 3 x 3 = 9, ab can be 9.

B) 14

Since 2 x 7 = 14, ab can be 14.

C) 21

Since 3 x 7 = 21, ab can be 21

D) 23

Since 23 is a prime number, there are no two prime numbers that can multiply to be 23. Thus, ab cannot be 23. (Recall that 1 is NOT a prime number.)

Answer: D

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