Question

Which of the following groups of numbers are all prime numbers? A. 2, 3, 5, 9 B. 2, 5, 15, 19 C. 3, 11, 23, 31 D. 7, 17, 29, 49

Answer 1

The correct answer is C: 3, 11, 23, 31. Each of these numbers has only two factors.

Why not A, B, D?
A - 9 is not a prime number (it has three factors: 1, 3, 9).
B - 15 is not a prime number (it has four factors: 1, 3, 5, 15).
D - 49 is not a prime number (it has three factors: 1, 7, 49).