We are short of clean safe drinking water, which is found to be the ______ in many third world countries.

A．example

B．event

C．case

D．object

哺乳类动物体内氨的主要去路是（）

A.渗入肠道

B.在肝中合成尿素

C.经肾泌氨随尿排出

D.生成谷氨酰胺

E.合成非必需氨基酸