如图所示，一轻质杠杆AC可绕支点O自由转动，在B点挂一重为G的物体．为使杠杆在水平位置保持平衡，画出施加在杠杆上最小力F的示意图．

北回归线自西向东依次经过的省级单位是：（   ）
①广西壮族自治区 ②云南省 ③广东省  ④台湾省

“身体分为头胸腹，两对翅膀三对足，头上一对感觉须，骨头长在肉外头”描写的是____ 的特征。

    A long time ago，there was a craftsman．His name was Lu Ban. One day，he was at work on a mountain．A leaf cut his finger．He saw many sharp teeth on the leaf．"Could I make a____with teeth like these? I could cut things with it ____，"he said to____．Soon he made a saw. It was the first____in the world．People are____using this tool today．