$15 \cdot 10$ Let the sets $P=\{x, 1\}, Q=\{y, 1,2\}$, where $x, y \in\{1$, $2, \cdots, 9\}$, and $P \subseteq Q$. Consider each ordered integer pair $(x, y)$ as a point, the number of such points is
(A) 9 .
(B) 14 .
(C) 15 .
(D) 21 .
(2nd "Hope Cup" National Mathematics Invitational Competition, 1991)
［Solution］According to the problem, when $x=2$, $y=3,4,5,6,7,8,9$; . When $x \neq 2$ (note $x \neq 1$) , $y=x=3,4,5,6,7,8,9$. In summary, there are 14 ordered pairs $(x, y)$. Therefore, the answer is (B).