—What is your brother like ？    —_________

A．He is fine, thank you.

B．He likes doing his homework now.

C．He is friendly and helpful.

D．He is a famous doctor.

用一氧化碳在加热的条件下还原氧化铜，发现已制得的铜又很快变黑了，你推测产生这种现象的原因是

A．温度太低

B．反应开始时，没有把试管内的空气排干净

C．试管口倾斜了

D．铜被空气中氧气所氧化