Because it's a square and doesn't list the 6 twice.
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1, 3 and 9
Square numbers 16, 25, 36, 49, 64, 81, 100
It's even
If you mean 'prime' factors, then an odd number of them CAN'T produce a perfect square. Consider 3 factors ... A, B, and C. If their product were a perfect square, then AxB=C, which can't be true if C is a prime number. For non-prime factors, an odd number of them may or may not be a perfect square: Three factors = not a square: 2 x 3 x 5 = 30. Three factors = a square: 2 x 3 x 6 = 36.
25 is odd