After a big flood,four-fifths of the farmer's crops ______.

问题描述:

After a big flood,four-fifths of the farmer's crops ______.
为什么填was ruined,不填were ruined?
帮忙翻译以下!

洪水过后,农民四分之三的庄稼都被毁了.
四分之三的庄稼 作为一个整体,用单数