23（）

单项选择题You will attend the course（）.

A．at the end of the year
B．in winter
C．in summer
D．whenever you wish

单项选择题Some parts of the Earth is warm and some are cold because（）.

A．a lot of the heat is reflected
B．there are clouds and dust in the air
C．different parts of the Earth receive and reflect different amount of heat
D．different parts of the Earth have different sources of heat