All of us have been here.
3．“a number of”是复数，修饰可数名词，谓语动词为复数，“the number of”也修饰复数名词，但谓语动词用单数。（黄金要点！！）
● A number of students have passed the exam.
● The number of students in this class is 50.
IV. 用作单数的复数形式主语：
1．如主语是指：time, money, weight, volume, etc. 虽为复数形式，但谓语用单数形式。
Two years is a short time.
One hundred dollars is needed by me.
2．下列单词如：physics，economics，mathematics，statistics，etc；measles，mumps，herpes，etc，news，ethics，politics，etc. 为复数形式，但谓语动词用单数。
Statistics is a dull subject.
Politics is important.
3．书、报、杂志、电影的名称作主语，谓动用作单数。
The New York Times is a good newspaper.