Populationhasbeenabigproblemsince20thcentury.Thereisn'tenoughfoodtofeedsuchabignumber.Thethingsnowisserious:developedcountryhasfewpeoplewhiledevelopingcountryhassomanypeople-evenbeyondthelimitofthenormallevel.Chinahasthelargestpopulation.Soitfacesthemostseriousproblem.
五十词.
应该是初中的吧
这样就差不多了.
要是再多的话就好象不大切合实际了.