用轻弹簧竖直悬挂质量为m的物体，静止时弹簧的伸长量为L现用该弹簧沿斜面方向拉住质量为2m的物体，系统静止时弹簧伸长量也为L斜面倾角为30°，如图所示.则物体所受摩擦力

把一个细胞中的磷脂分子提取出来，在空气﹣水界面上将它们铺成单分子层．推测在下列生物中，水面上磷脂单分子层面积与细胞表面积之比最小的是（　　）

我市某中学八年级一班准备在“七一”组织参加红色旅游，班长把全班48名同学对旅游地点的意向绘制成了扇形统计图，其中“想去我市龙州县红八军纪念馆参加的学生数”的扇形圆心角为60°，则下列说法中正确的是（   ）

如图甲所示为一款能自动清扫地面的扫地机器人。机器人通过电动机旋转产生高速气流，将灰尘、杂物吸入集尘盒；其防滑轮皮采用凸凹材质制成；底部安装有塑料刷，用于清扫吸附在地板上的灰尘及轻小物体；前端装有感应器，通过发射、接收超声波或红外线来侦测障碍物。当剩余电量减为电池容量的20%时，机器人会主动寻找充电器充电。表一为某扫地机器人的部分参数。电池容量指放电电流与放电总时间的乘积。

表一：

机器人中吸尘电机的吸入功率是衡量其优劣的重要参数，测得某吸尘电机的吸入功率与真空度、风量间的对应关系如表二。真空度指主机内部气压与外界的气压差。风量指单位时间内通过吸尘电机排出的空气体积。吸尘电机吸入功率与输入功率的比值叫做效率。

表二：

     We have no idea about  1 men first began to use salt. But we do know that it had been used in many different ways 2 the history.
For example, in many history books it is 3that people who lived three thousand years ago 4salted fish. Thousands of years ago in Egypt（埃及）, salt 5 keep the dead. It can keep the dead bodies6decay.（腐烂）
   In the eighteenth century, a person who stole salt was 7 to have broken the 8. if a person was caught stealing salt, he would be thrown into 9. Books also record that in England about ten thousand people were put into prison during that century10stealing salt! In the year 1553, if a man took 11than his share of salt, he would be thought to have broken the law and would be12 punished(惩罚).
   oSalt was very13 on the dinner table of a king. It was always put14 the king when he sat down to eat. Then important visitors sat near the salt15 less important visitors were given seats farther away from it.