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问题 选择题

As these beautiful places attract lots of tourists, their protection comes under ______.[ ]

A. threat

B. control

C. repair

D. construction

答案

答案：A

解答题
已知双曲线
x2
a2
-
y2
b2
=1(a＞0，b＞0)的右准线l2与一条渐近线l交于点P，F是双曲线的右焦点．
（Ⅰ）求证：PF⊥l；
（Ⅱ）若|PF|=
2
，且双曲线的离心率e=
3
，求该双曲线的方程；
（Ⅲ）若过点A（2，1）的直线与（Ⅱ）中的双曲线交于两点P1，P2，求线段P1P2的中点M的轨迹方程．
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选择题

— Is there any possible way to help the children get rid of such a bad habit?

— To be honest, it's very hard, but we ____ on this problem. [ ]

A. work

B. worked

C. are working

D. were working

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