5. Given the set $A=\left\{x \mid x^{2}-2 x-3=0\right\}, B=\{x \mid a x=1\}$. If $B \subseteq A$, then the product of all possible values of the real number $a$ is ( ).
A. -.1
B. $-\frac{1}{3}$
C. 0
D. None of the above
5. C. When $a=0$, $B=\varnothing \subseteq A$; when $a \neq 0$, $B=\left\{\frac{1}{a}\right\}$, then $\frac{1}{a}=-1$ or 3, i.e., $a=-1$ or $\frac{1}{3}$.