皮带打滑的原因是（）。
A．磨擦力大
B．磨擦力小
C．无磨擦力
参考答案：B
On the annual International Volunteers Day, the world _____ the work of millions of people who give their time to help others.
A．abolishes
B．accompanies
C．acknowledges
D．appeals
运行中为休要注意燃烧的调整？
