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答案揭晓：
A sea of
大量
例句：
The square was a sea of people.
广场上人山人海。
A sea of faces:
人山人海
例句：
There was a sea of faces on every scenic spot in China during Spring Festival.
春节期间，中国的每个景点都是人山人海。
A crowd of:
一大群；一群
例句：
A crowd of youths started smashing windows.
一群年轻人开始砸窗户。
Assignment（脑洞大开时间）
Lame duck
Song for you（片尾曲）
FrogMonster - Remember Our Summer