若(a-1)x2+bx+c=0是关于x的一元二次方程,则( )
A.a≠0
B.a≠1
C.a≠-1
D.a=1
由题意得:a-1≠0,
解得:a≠1,
故选B.
某校检查学生作业时,抽出每班学号尾数为5的学生作业进行检查,运用的抽样方法是( )
A.分层抽样
B.抽签抽样
C.随机抽样
D.系统抽样
The number of books in the bookshop is about 10,000 and a number of them _______ about science.[ ]
A. is
B. was
C. are
D. were