题目

一物体运动的v﹣﹣t图象如图所示，设向右为正，下列关于前4s内物体运动情况的判断，下列说法正确的是（     ）                                                                                                                                                                                                                           A．  物设圆C的圆心为双曲线的左焦点，且与此双曲线的渐近线相切，若圆C被直线l：x-y+2=0截得的弦长等于，则a等于A.1B.C.D.4