为什么用world'speacekeeping?而不是keepi?
a plays an important role in world's peace keeping.
应该说两者都可。只是world's peace keeping是名词短语，而keeping the peace of the world是动名词短语。英语中有一个习惯，能用名词的情况尽量用名词。如：We should spend more time on our English study.而少用 We should spend more time in learning English.