1. Given that $f(x)$ is an even function, and
$$
f(2+x)=f(2-x) \text {, }
$$

when $0 \leqslant x \leqslant 2$, $f(x)=3^{-x}$. If $n \in \mathbf{N}_{+}, a_{n}=$ $f(n)$, then $a_{200}$ equals $(\quad)$.
(A) 27
(B) 3
(C) $\frac{1}{3}$
(D) $\frac{1}{27}$
-1.C.
Since $f(2+x)=f(2-x)$, we have
$$
\begin{array}{l}
f(4+x)=f(2+(2+x)) \\
=f(2-(2+x))=f(-x) .
\end{array}
$$

Also, since $f(x)$ is an even function, $f(-x)=f(x)$, thus
$$
f(4+x)=f(x) \text {. }
$$

Therefore, we know that $f(x)$ is a periodic function with a period of 4. Hence,
$$
\begin{array}{l}
a_{2007}=f(2007)=f(501 \times 4+3)=f(3) \\
=f(-3)=f(-3+4)=f(1)=\frac{1}{3} .
\end{array}
$$