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Correct Answer: C
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A prime factor is a prime number, a number that can be divided only by itself and 1. The prime numbers are 2, 3, 5, 7, 11, and so on. (Notice that 0 and 1 aren’t prime numbers.) To factor down a number ending in 0, divide by 10. For Column A, that gives you 121 × 10. Theoretically you’ve memorized your perfect squares and know that 121 = 11 × 11. For Column B, dividing by 10 gives you 1,210 × 10. Divide by 10 again to get 121 × 10 × 10.
Because 10 factors down into 2 and 5, the greatest prime factor of both columns is 11.
Correct Answer: C.