题目

 Tom sat under a tree and seeing his friend, ___ up in no time. A. to stand    B. standing    C. stood    D. would stand 答案：C与y轴相切和半圆x2＋y2＝4（0≤x≤2）内切的动圆的圆心的轨迹是(　　) A．y2=4(x-1) (0<x≤1)   B．y2=-4(x-1)(0<x≤1) C．y2=4(x+1) (0<x≤1) D．y2=-2(x-1) (0<x≤1)