be careful when crossing the street!A. MustB. ShouldC. Ought toD. Do

问题补充：

be careful when crossing the street!

A. Must

B. Should

C. Ought to

D. Do

答案：

A. Must

B. Should

C. Ought to

D. Do

解析：加强祈使句的语气,不管动词是系动词还是行为动词,都在动词前加do。