下列哪项不属于环境介质（）
A.大气
B.水体
C.土壤
D.生物体
E.有机和无机物
参考答案：E
—Let's go to see a comedy.
— _____.[ ]
A. Excuse me.
B. That's great.
C. Excuse me.
D. How do you do?
试述胎盘屏障有何生理作用？
