1. Given that the function $f(x)$ is an odd function on $\mathbf{R}$, and $g(x)$ is an even function on $\mathbf{R}$. If $f(x)-g(x)=$ $x^{2}+9 x+12$, then $f(x)+g(x)=(\quad)$.
(A) $-x^{2}+9 x-12$
(B) $x^{2}+9 x-12$
(C) $-x^{2}-9 x+12$
(D) $x^{2}-9 x+12$
$-1 . A$
From $f(x) 、 g(x)$ being odd and even functions respectively, we know
$$
f(-x)=-f(x), g(-x)=g(x) .
$$

Substituting $-x$ for $f(x)-g(x)=x^{2}+9 x+12$, we get $f(-x)-g(-x)=x^{2}-9 x+12$.