12．在平面直角坐标系中，两点 $P_{1}\left(x_{1}, y_{1}\right), P_{2}\left(x_{2}, y_{2}\right)$ 间的＂L－距离＂定义为 $\| P_{1} P_{2}\left|=\left|x_{1}-x_{2}\right|+\left|y_{1}-y_{2}\right|\right.$ ．则平面内与 $x$ 轴上两个不同的定点 $F_{1}, F_{2}$ 的＂L－距离＂之和等于定值。（大于 $\| F_{1} F_{2} \mid$ ）的点的轨迹可以是
A
D
C
D
Answer key
$A$