题目

I often wash my clothes, water the flowers and clean the rooms on the weekend. I’m ________. ( )A.helpful B.shy C.polite 答案：【答案】A【解析】句意：我经常在周末洗衣服，浇花和打扫房间。我是_____。helpful有帮助的，shy害羞的，polite有礼貌的。根据句意可知，helpful符合要求。故选A。在水平桌面上有一重为12N的长方体，用4N的水平拉力向右拉，长方体静止不动，此时它受到的摩擦力为________N；拉力增大至6N时长方体做匀速直线运动此时它受到的摩擦力为________N；若拉力增大至8N时长方体所受的摩擦力为________N．