----you understand this rule,you will have no further diffic

此题不很好,我认为AC都可以选,不用过分分析了.since表示原因的语气比较一般,because最强,as最弱（用于告诉众所周知的原因）,回答why的只能用because.例如：As China has a large population...由于...这个是大家都知道的原因,语气较弱,所以用as和since,如果告诉的是不为人知的原因,多用because He was late for the meeting,because he...