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. Given that $f(x)$ and $g(x)$ are odd and even functions respectively, we know
$$
f(-x)=-f(x), g(-x)=g(x) \text {. }
$$

In $f(x)-g(x)=x^{2}+9 x+12$, substituting $-x$ for $x$, we get
$$
f(-x)-g(-x)=x^{2}-9 x+12 \text {, }
$$

From this, we obtain $f(x)+g(x)=-x^{2}+9 x-12$.