原式=2a-a-1+a+1
=a-1+a+1
=2a；
当a=0（a≠1）时，原式=0．
Last year the number of students who graduated with a driving license reached 200,000, a (n) _____ of 40,000
per year. [ ]
A. average
B. number
C. amount
D. quantity
下列税种中属于商品课税的是（）。
A、增值税
B、消费税
C、企业所得税
D、关税
E、个人所得税
