在标准状况下，相同体积的下列气体所含物质的质量最小的是[ ]
A．O2
B．N2
C．CH4
D．C2H2
答案：C
当肾小管液流过近端小管时，其中（）
A.K+浓度升高
B.Na+浓度降低1/3
C.渗透压基本不变
D.渗透压降低
E.渗透压升高
A "Backgrounder" permits newspapermen to publish information given them though without attribution to the source.
A. a person who remains behind the sceneB. a person providing the background knowledgeC. a press conferenceD. a news agency
