3. If $\left(\log _{2} 3\right)^{x}-\left(\log _{5} 3\right)^{x} \geqslant\left(\log _{2} 3\right)^{-y}-\left(\log _{5} 3\right)^{-y}$, then
(A) $x-y \geqslant 0$
(B) $x+y \geqslant 0$
(C) $x-y \leqslant 0$
(D) $x+y \leqslant 0$
(B)
3. 【Analysis and Solution】Let $f(t)=\left(\log _{2} 3\right) t-\left(\log _{5} 3\right) t$, then $f(t)$ is a strictly increasing function on $\mathbf{R}$ - the original inequality is $f(x) \geqslant f(-y)$. Therefore, $x \geqslant -y$, which means $x+y \geqslant 0$.