答案：D

题目分析：考查动词remain用法。动词remain可以转换成remaining剩下的；固定搭配sth remain to be done某事有待于被做；形容词remaing和remain to be done合在一起形成形容词短语remaining to be done修饰前面的名词。句意：—怎么了？—有很多问题有待于被解决。故D正确。

点评：动词remain可以作为系动词也可以作为行为动词，用法较为复杂。remain可以转换成remaining剩下的；固定搭配sth remain to be done某事有待于被做；形容词remaing和remain to be done合在一起形成形容词短语remaining to be done修饰前面的名词。