1. Let $M=\left\{a \mid a=x^{2}-y^{2}, x, y \in \mathbf{Z}\right\}$. Then the correct option is ( ).
(A) $9 \in M, 10 \in M$
(B) $9 \notin M, 10 \in M$
(C) $9 \in M, 10 \notin M$
(D) $9 \notin M, 10 \notin M$
- 1. C.

Since $9=3^{2}-0^{2}$, we have $9 \in M$.
Assume $10 \in M$. Then there exist integers $m, n$ such that
$$
10=m^{2}-n^{2}=(m+n)(m-n) \text{. }
$$

Decomposing 10 into the product of two integer factors, one factor must be odd and the other even, which contradicts the fact that $m+n$ and $m-n$ have the same parity.
Thus, $10 \notin M$.