某同学测量一个圆柱体的电阻率，需要测量圆柱体的尺寸和电阻．

①分别使用游标卡尺和螺旋测微器测量圆柱体的长度和直径，某次测量的示数如图 （a）和如图 （b）所示，长度为____ cm，直径为____ mm．

②按如图 （c）连接电路后，实验操作如下：

（a）将滑动变阻器R₁的阻值置于最____处（填“大”或“小”）；将S₂拨向接点1，闭合S₁，调节R₁，使电流表示数为I₀；

（b）将电阻箱R₂的阻值调至最____（填“大”或“小”），S₂拨向接点2；保持R₁不变，调节R₂，使电流表示数仍为I₀，此时R₂阻值为1280Ω；

③由此可知，圆柱体的电阻为____Ω．

    Every day in China, about 200 million children go to school. Many of them take school buses to school and back home. Most of the time, the school buses are very safe for them. But sometimes they are also dangerous.

    Several years ago, a serious accident happened to a nine – seat minibus in November in Gansu. There were 62 children inside, and 21 of them died.

    What should we do to keep us safe on the road? First, we should wait for the school bus on time at the right place when we are going to school or coming back home. Second, we shouldn't run or fight on the bus when it is going. Third, we mustn't try to get off when it is moving. Last, don't get on the bus if it is overloaded(超载).

    Our government(政府)also makes rules for school buses. Under the new rules, school buses have a speed limit of 80km per(每)hour on highways and 60km per hour on normal roads. While on the road, other cars must let school buses go first.

根据材料内容选择最佳答案。