Which of the following numbers is prime?

Which of the following numbers is prime?

• A. 4
• B. 8
• C. 11
• D. 12

Answer: (C)

Prime numbers are numbers greater than 1 that have exactly two distinct positive divisors: 1 and the number itself. 11 fits this because it’s only divisible by 1 and 11; it’s odd, so it’s not divisible by 2, and the sum of its digits is 2, so it’s not divisible by 3. Since no integer up to the square root of 11 divides it, 11 is prime. The other options aren’t prime because they have additional divisors beyond 1 and themselves: they’re all even and greater than 2, so each is divisible by 2 (and, in the case of the larger ones, by other numbers as well), making them composite.

Prime numbers are numbers greater than 1 that have exactly two distinct positive divisors: 1 and the number itself. 11 fits this because it’s only divisible by 1 and 11; it’s odd, so it’s not divisible by 2, and the sum of its digits is 2, so it’s not divisible by 3. Since no integer up to the square root of 11 divides it, 11 is prime. The other options aren’t prime because they have additional divisors beyond 1 and themselves: they’re all even and greater than 2, so each is divisible by 2 (and, in the case of the larger ones, by other numbers as well), making them composite.