7. Let the set $A=\left\{n \left\lvert\, \frac{n}{3} \in \mathbf{N}_{+}\right.\right\}$,
$$
B=\left\{y \mid y=x+4+\sqrt{5-x^{2}}\right\} .
$$

Then the number of elements in the set $A \cap B$ is ().
(A) 1
(B) 2
(C) 3
(D) 4
7. B.

Let $x=\sqrt{5} \sin \theta\left(-\frac{\pi}{2} \leqslant \theta \leqslant \frac{\pi}{2}\right)$.
Then $y=\sqrt{5} \sin \theta+4+\sqrt{5} \cos \theta$
$=4+\sqrt{10} \sin \left(\theta+\frac{\pi}{4}\right)$.
By $-\frac{\pi}{4} \leqslant \theta+\frac{\pi}{4} \leqslant \frac{3 \pi}{4}$
$\Rightarrow-\frac{\sqrt{2}}{2} \leqslant \sin \left(\theta+\frac{\pi}{4}\right) \leqslant 1$
$\Rightarrow 4-\sqrt{5} \leqslant y \leqslant 4+\sqrt{10}$.
Also, $A=\{3,6,9, \cdots\}$, then $A \cap B=\{3,6\}$, i.e., $A \cap B$ contains two elements.