设随机变量 $X_i \sim\left(\begin{array}{ccc}-1 & 0 & 1 \\ \frac{1}{4} & \frac{1}{2} & \frac{1}{4}\end{array}\right)(i=1,2)$, 且满足 $P\left\{X_1 X_2=0\right\}=1$, 则 $P\left\{X_1=\right.$ $\left.X_2\right\}$ 等于
A
0.
B
$\frac{1}{4}$.
C
$\frac{1}{2}$.
D
1 .
E
F