题目

Tom, together with his parents, often ________ some cleaning in their house on Sundays. A. do     B. does       C. is       D. are 答案：B地面上有一条大木杆，抬起A端需用力300N，抬起B端需用力200N．这条木杆的    端较粗，整个木杆的重量（所受的重力）为    N．