口服药物后，进人体循环有效量减少的现象是（）。 A.恒比消除 B.药物诱导 C.首关

The father is good             stories.

A．at telling

B． for speaking

C．at talking

D．at saying

弹奏钢琴上B调的“1”音键与C调的“1”音键相比     调“1”音键弹奏的钢

丝振动的频率大。

John is very _____ －if he promises to do something he'll do it. [ ]

A. independent

B. confident

C. reliable

D. flexible

成语“秦晋之好”当中有我国哪两个省份的简称[ ]

A、陕西、河北

B、陕西、山西

C、云南、山西

D、甘肃、河北

简述边际成本与平均成本的关系？