3. The sides of a right-angled triangle are $a-b, a, a+b$, and both $a$ and $b$ are positive integers. Then one of the sides could be ( . .
(A) 61
(B) 71
(C) 81
(D) 91
$3 . \mathrm{C}$
3. Since only 81 is a perfect square among the 4 options, it is the only one that could possibly be a side of a right triangle.