The boy is said ________ across the busy road when suddenly a car cut in and knocked him over. [ ]

A. to be cycling    

B. to have cycled

C. to have been cycled    

D. to cycle

心包腔内液体可起到的主要生理作用是

A．防止感染

B．参与心肌电传导

C．营养心肌

D．体液免疫

E．润滑