变形虫是很简单的生物体.The amoeba is a very simple organism.当环境不利时,变形虫产生包囊.An amoeba forms a cyst when conditions are unfavourable.变形虫, 一种淡水生物, 就是这种情况.The amoeba, a denizen of fresh water, is such a case....
土著人以前总是用沉重的石头把这种金属锤打成箭头.The natives used to hammer the metal into arrowheads using heavy stones.来访者用拳头使劲地锤打着大门.The visitor hammered against the door with his fists.我们用铁锤锤打金属.We hammer metal with a hammer....
It is gravitation that causes the apple to fall.是地心吸引力使苹果下落.We owe to Newton the principle of gravitation.万有引力原理的发现应归功于牛顿.The law of gravitation explains many phenomena not previously, understood.引力定律解释了许多以前所不能理解的现象....