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
7. B.

Notice,
$$
\begin{array}{l}
\frac{1}{\sqrt{x}}-\frac{1}{\sqrt{y}}=\frac{1}{\sqrt{45}} \Leftrightarrow \frac{1}{\sqrt{5 x}}-\frac{1}{\sqrt{5 y}}=\frac{1}{15} \\
\Rightarrow \frac{1}{5 x}=\frac{1}{225}+\frac{1}{5 y}+\frac{2}{15 \sqrt{5 y}} \\
\Rightarrow \sqrt{5 y} \in \mathbf{Q} .
\end{array}
$$

Similarly, $\sqrt{5 x} \in \mathbf{Q}$.
Let $x=5 a^{2}, y=5 b^{2}\left(a, b \in \mathbf{Z}_{+}\right)$.
Then $\frac{1}{\sqrt{x}}-\frac{1}{\sqrt{y}}=\frac{1}{\sqrt{45}} \Leftrightarrow \frac{1}{a}-\frac{1}{b}=\frac{1}{3}$
$$
\Leftrightarrow(a, b)=(2,6) \text {. }
$$

Also, $(a, b)$ corresponds one-to-one with $(x, y)$, therefore, the number of elements in set $M$ is 1.