古生界自下而上可分为寒武系、自留系、泥盆系、石炭系、二叠系，均属（）地层单位。
参考答案：年代
Patty: How about going for a walk?
Nancy: ________.
A．I’ll walk with you at eight
B．That’s not a bad idea
C．Walking is a kind of exercise
D．It will take me minutes to have a walk
填料
