**backsolve**. We have to look at the answer choices, and figure out which one has 3 integer factors: itself, its square root, and 1. Since EVERY choice is going to have the factors 1 and itself, we're basically looking to see which ones are perfect squares, and then see which don't have any other factors.

Start by eliminating 33. That's not a perfect square. Then eliminate 64 and 100, because they're even, so they're definitely going to have 2 as a factor. Then you're down to 121 and 81.

121 is 11

^{2}, and 81 is 9

^{2}. Can you think of any factors of 121 other than 1, 11, and 121? No? How about 81? Any factors other than 1, 9, and 81? How about 3? Yep, 3 is also a factor, so we can eliminate 81. We're left with only 121.

**Choice A.**

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