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20 10 the math problem in the senior high school entrance examination 12
A and d are excluded because the square number must be the square of a certain number.

The square of 5 is 25.

Square of 35 1225

The rule of triangle number is y (n) = (1/2) * N2+(1/2) * n.

25 obviously does not belong to the number of rows in a triangle.

So the answer is d.