the trees must _______(water)three times a week at first.
问题描述:
the trees must _______(water)three times a week at first.
答
be watered这里是被动语态,前有情态动词,所以be为原型,water是动词“浇水”,改作过去分词形式.首先这些树每星期必须被浇三次水.