A:that; what
B:that; which
C:what; which
D:which; that
—When do you do__________ homework?
—I often do ________ homework in the evening.
已知圆C:x2+y2+4x﹣2y+a=0,直线l:x﹣y﹣3=0,点O为坐标原点.
求过圆C的圆心且与直线l垂直的直线m的方程;