简述三种常用的天球坐标系。
参考答案：
地平坐标系、赤道坐标系和黄道坐标系。
—Would you like to go out for a walk ?
— .
A．No, I don’t feel in the mood for it now.
B．No, but I can’t afford the time.
C．You may ask your sister to go, too.
D．No, thank you.
We can never expect________bluer sky unless we create________less polluted world.
A．a; a
B．a; the
C．the; a
D．the; the
