《素问.上古天真论》曰“筋骨坚，发长极，身体盛壮”。是女子哪个年龄段的生理表现（）。

A."二七"

B."三七"

C."四七"

D."五七"

E."六七"

I haven't seen Anna since she left our hometown two years ago, but we do keep in touch with each other

by sending emails ________.[ ]

A．here and there  

B．sooner or later

C．at one time  

D．now and then