表面渗碳时，钢件一般要加热到（）。
A.500℃~650℃
B.750℃~850℃
C.900℃~950℃
D.450℃~500℃
参考答案：C
雌二醇
A．四氮唑比色法B．铁-酚试剂比色法C．异烟肼比色法D．气相色谱法E．碘量法下列药物可采用的含量测定方法为
She ____________ as an animal trainer since 2003.
A．has worked
B．works
C．will work
D．worked
