There are traffic lights at many crossroads. Traffic lights are important. They tell you when to go and when to stop. When the green light is on, you can go. When the red light is on, you must not go. You must stop. You watch the lights. Now the green light is on again. You can go.

1.There are traffic lights ________.

A at homes

B in parks

C on rivers

D at crossroads

解析：选D。D 细节理解题。根据There are traffic lights at many crossroads.可知许多十字路口有交通灯，故选D。

2.Traffic lights are ________.

A beautiful

B important

C magic

D clever

解析：选B。B 细节理解题。根据Traffic lights are important.可知交通灯很重要，故选B。

3.You must stop when the ________ light is on.

A green

B yellow

C red

D blue

解析：选C。C 细节理解题。根据When the red light is on, you must not go. You must stop.可知红灯亮的时候，不能走，必须停下来。故选C。

4.When the green light is on, you ________.

A must stop

B watch the lights

C must not go

D can go

解析：选D。D 细节理解题。根据When the green light is on, you can go.可知绿灯亮的时候，可以走，故选D。