7. Let the set
$$
M=\left\{(x, y) \left\lvert\, \frac{1}{\sqrt{x}}-\frac{1}{\sqrt{y}}=\frac{1}{\sqrt{45}}\right., x, y \in \mathbf{Z}_{+}\right\} \text {. }
$$

Then the number of elements in the set $M$ is ().
(A) 0
(B) 1
(C) 2
(D) 3