已知随机变量 $X \sim\left(\begin{array}{cc}0 & 1 \\ \frac{1}{4} & \frac{3}{4}\end{array}\right), Y \sim\left(\begin{array}{cc}0 & 1 \\ \frac{1}{4} & \frac{3}{4}\end{array}\right), E X Y=\frac{5}{8}$, 则 $P\{X+Y \leqslant 1\}$ 等于
A
$\frac{1}{8}$.
B
$\frac{1}{4}$.
C
$\frac{3}{8}$.
D
$\frac{1}{2}$.
E
F