1. (2002 Anhui Province Competition Question) Given the sets $P=\left\{x \mid x^{2}=1\right\}$ and $Q=\{x \mid m x=1\}$. If $Q \subset P$, then the number of possible values for the real number $m$ is ( ).
A. 0
B. 1
C. 2
D. 3
1. Choose D. Reason: Since $P=\{-1,1\}, Q \subset P$, then $Q$ could be $\varnothing,\{-1\},\{1\}$. When $Q=\varnothing$, $m=0$; when $Q=\{-1\}$, $m=-1$; when $Q=\{1\}$, $m=1$, hence $m$ has 3 values: $0,-1,1$.