题目

 When Miss Zhao came into the classroom, the students___. A. talked B. were talking C. talking D. talk 答案：B如图所示，水平桌面上叠放着A、B两物体，B物体受力F作用，A、B一起相对地面向右做匀减速直线运动，则B物体的受力个数为  （     ） A．4个                 B．5个     C．6个                D．7个