题干
—Could I swim here?
— ．Look at the sign NO SWIMMING．
A：No，you couldn't
B：Certainly，you could
C：Sorry，please don't
D：Of course，you can
答案(点此获取答案解析）
C
—Could I swim here?
— ．Look at the sign NO SWIMMING．
A：No，you couldn't
B：Certainly，you could
C：Sorry，please don't
D：Of course，you can
与定点 
的距离和它到定直线 
的距离的比是 
∶ 
，记点 
的轨迹为 
.