【单选题】Chinese is one of five official languages in ______ .
【判断题】锆石属四方晶系,是非均质体,二轴晶宝石。
【单选题】What's the main topic of the passage?
A.
The current crisis of the U. N.
B.
Suggestion on changes in working practices of file U. N.
C.
Improvement in the efficiency of the U. N.
D.
The U. N. in the Iraq war.
【单选题】为了得到一个由电压控制的电流源,应选择( )负反馈放大电路。