肾脏的机能是（）
A.排泄代谢产物
B.维持酸碱平衡
C.调节水平衡
D.分泌生物活性物质
E.以上都不是
参考答案：A, B, C, D
The shortage of rainfall this summer in the region is sure to cause ______ of vegetables for the food market.
A．scarcity
B．variety
C．rarity
D．validity
