The river near our school u________ to be clean,but now it becomes dirty.
问题描述：
The river near our school u________ to be clean,but now it becomes dirty.
答
used
短语知识：used to 意思是 “过去是……”
翻译：我们学校附近的河曾经是清澈的,但是现在变的浑浊了.