( )enough water to the people in the city is a problem(supply) 用所给词的适当形式填空

问题描述:

( )enough water to the people in the city is a problem(supply) 用所给词的适当形式填空
怎么填啊,要说明原因

动名词,作主语能带to么不可以,to表示一个原因