number of mass transfer unit

例句：

1. 1.The number of students in the class is increasing.
   班上学生的数量正在增加。
2. 2. The number of cars on the road has doubled since last year.
   路上的汽车数量从去年增加了一倍。
3. 3. The number of people who signed up for the event was impressive.
   报名参加该活动的人数令人印象深刻。