The police were seeking more information to find out _____ the rich merchant.   [ ]

A. who was it that killed                

B. who it was that killed    

C. it was who killed                    

D. who was it killed

关于变速直线运动的平均速度公式v=s/t理解，其中正确的是[ ]

A.v的大小与s、t的选取无关

B.能够准确地反映物体运动的情况

C.在任何情况下，v是不变的

D.必须指明在哪段路程或某段时间内才有意义