Which number is prime?

Which number is prime?

• A. 7
• B. 9
• C. 1
• D. 12

Answer: (A)

Prime numbers are numbers greater than 1 that have exactly two distinct positive divisors: 1 and the number itself. The number seven has divisors only 1 and 7, so it is prime. The other options don’t fit: nine is divisible by 1, 3, and 9; one has only one divisor (itself); twelve is divisible by several numbers (1, 2, 3, 4, 6, and 12).

Prime numbers are numbers greater than 1 that have exactly two distinct positive divisors: 1 and the number itself. The number seven has divisors only 1 and 7, so it is prime. The other options don’t fit: nine is divisible by 1, 3, and 9; one has only one divisor (itself); twelve is divisible by several numbers (1, 2, 3, 4, 6, and 12).