曼昆经济学原理中有这么一段话：
china,with fat trade surpluses and bulging foreign-exchange reserves,is buying u.S. governmeng secutities,especially Treasury bonds and bonds issued Fannie Mae and Freddie Mac. that is good for America . such investment add liquility to the U.S.housing market and help hold down U.S. interest rates.
请问下，中国持有房利美和房地美的债券，可以增加美国的住宅市场的流动性和有利于降低美国的利率。这话怎么理解？特别是对于保持美国的低利率这因果关系，求分析过程。。。。