If it were not for oxygen and nitrogen in the air, no man could live on the earth. 大气中如果没有氧和氮,人就无法生存于地球了。
There are millions of living things on the earth. 世界上有数以百万计的生物。
Long ago enormous animals lived on the earth. 很久以前,地球上生活着巨大的动物。
Scientists have set up two kinds of relay stations. One kind is on the earth, the other is a satellite placed in space around the earth. 科学家们已建立了两种中继站,一种在地球上,而另一种是放在地球周围太空中的卫星上。