会计档案的保管期限，根据其特点分为永久性和定期性两类。( )
参考答案：对
—Look, there_____ many birds around here, but now we hardly see them. What a pity!
—Yeah! The air pollution has caused the problem!
A．are
B．used to have
C．used to be
D．use to be
在酸化中常用的互溶剂为（）。
A、乙酸
B、SP169
C、乙二醇单丁醚
D、丙酮
