Is that the reason ________he explained to all at the meeting for his carelessness in his work?  Actually we think he is a man who is hard ________ himself especially in work.

A．that; on

B．what; with

C．why; at

D．which; to

在抽真空的容器中加热固体NH₄C1（s），有一部分分解成NH₃（g）和HCl（g），当体系建立平衡时，其独立组分数c和自由度数f是（）。

A、c=l，f=l；

B、c=2，f=2；

C、c=3f=3；

D、c=2f=1