Concept:A prime number has exactly two factors: 1 and itself. 1 is not prime, and 2 is the only even prime.
Explanation:List all positive prime integers less than 50:
2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47.
Count them: there are 15 numbers in this list.
Answer:D. 15