题目

第三节 完形填空（共20小题；每小题1分，满分20分） 阅读下面短文，掌握其大意，然后从31-50各题所给的四个选项（A、B、C和D）中，选出最佳选项, 并在答题卷上将该项涂黑。 People often fall ill because of me.   31__, they can hardly blame me; it is largely their own  32  . A tired person may get caught,  33 _ when he goes to crowded places with polluted air. A sudden change in 34   is another factor. In hot summer, people turn on the air-conditioner upon returning home. They will catch a cold easily. My latest victim is an ener（本小题满分14分）在平面直角坐标系中,设点,直线:,点在直线上移动,是线段与轴的交点, ．（I）求动点的轨迹的方程；（II）设圆过,且圆心在曲线上,是圆在轴上截得的弦,当运动时弦长是否为定值？请说明理由．