设二维随机变量 $(X, Y)$ 的概率密度为
$$
f(x, y)= \begin{cases}2 e^{-(2 x+y)}, & x>0, y>0, \\ 0, & \text { 其他. }\end{cases}
$$
求: (1) $f_{X \mid Y}(x \mid y), f_{Y \mid X}(y \mid x)$;
(2) $P\{X \leqslant 2 \mid Y \leqslant 1\}$.
$\text{A.}$
$\text{B.}$
$\text{C.}$
$\text{D.}$