what's your first impressions of Beijing．impression为什么要加S呢

留下第一印象的事情,印象本来就[C][U]；影响、效果的时候不可数.见eg:
印象[C][U][(+on)]
The new teacher made a good impression on the students.
新教师给学生留下了一个好印象.
模糊的观念;感想[C][+that]
I have the impression that I've seen that man before.
我觉得我以前见过那个人.
影响;效果[U][(+on)]
My advice seemed to make little impression on him.
找了几句例句参考.
People are liable to judge others from their own first impressions.人们容易用他们自己的最初印象来评价别人.It is dangerous to judge on the basis of first impressions.凭藉初次印象进行判断是危险的.
其实我觉得first impressions 约定俗成了.第一印象就是很多方面构成的,就习惯复数- -
再问： 这是英语课本里面的句子，不可能错的呀