一道英语改句子的题目!

问题描述:

一道英语改句子的题目!
On their way to school,they saw a nice watch on the street corner.(保持句意相同)
a nice watch on the street corner _______ their _______on their way to school.
我填的答案是“caught ,eyes”,答案上写的是“caught ,eye”
好奇怪,eye是可数名词,而且有their!

catch one's eye 是固定搭配,eye 在这里指的是目光(吸引某人的目光),所以在此不能用复数.就好比我们一般都说The girl who has long hair is my sister.这里的hair 只能用不可数的形式,如果加了s就变成她只有可数的几根头发了.