My new classmate told us about his trip to _____ Great Wall. He said it was _____ exciting experience for him. [ ]

A. the; an

B. /; the

C. the; /

D. /; a

预防医学研究的人类环境主要包括( )

A．空气、水、土壤
B．空气、水、土壤、食物
C．人类的生活和生产环境
D．人类的自然环境和社会环境
E．人类的城市环境