如图所示的实例中,是为了减小有害摩擦的是
A.轴承中装有滚珠
B.汽车轮上装有防滑链
C.汽车轮胎上制有凹凸不平的花纹
D.运动鞋底做成凹凸不平的花纹
答案:A
_____ is known to us all is that the 2012 Olympic Games will take place in London.[ ]
A. It
B. Which
C. As
D. What
花费于生产后代和抚育后代的能量和物质资源称为______。