Population has been a big problem since 20th century .There isn't enough food to feed such a big number.The things now is serious :developed country has few people while developing country has so many people - even beyond the limit of the normal level.China has the largest population.So it faces the most serious problem .
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.