质量为m，带电量为q的小物块，从倾角为θ的光滑绝缘斜面上由静止下滑，整个斜面置于方向水平向里的匀强磁场中，磁感应强度为B，如图所示，若带电小物块下滑后某时刻对斜面的作用力恰好为零，下面说法中正确的是（   ）

    Tom is seven . He goes to school every day .The school is near his home , so he goes there on foot and comes back home on time . But today he is late .his mother asks his , “Why are you late today?”“I am in the headmasters office?”“Why do you go the headmaster's office?”“Because my teacher asks us a question in the class , nobody (没人)can answer(回答)it , but I can .“It's good to answer the teacher's question.”“But the question is who pus ink on my chair.”