1. Let $S=\{(x, y) \mid x y>0\}, T=\{(x, y) \mid$ $x>0$ and $y>0\}$. Then ().
(A) $S \cup T=S$
(B) $S \cup T=T$
(C) $S \cap T=S$
(D) $S \cap T=\varnothing$
$$
\begin{array}{l}
\text { I.1.A. } \\
\text { Given } S=\{(x, y) \mid x y>0\} \\
=\{(x, y) \mid x>0, y>0\} \cup\{(x, y) \mid x<0, y<0\},
\end{array}
$$

we know that $T \subset S$.