人工合成的立方氮化硼（BN），其硬度已超过金刚石。氮化硼中氮元

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问题选择题

人工合成的立方氮化硼（BN），其硬度已超过金刚石。氮化硼中氮元素的化合价为-3价，则硼元素的化合价为（　　）

A．－3

B．0

C．＋1

D．＋3

答案

答案：D

题目分析：根据化合物中正负化合价代数和为零的原则进行计算。

选择题

During the war, ＿＿＿＿but also he lost his wife and his child.

A．not was his job in the lab taken away

B．not only was his job in the lab taken away

C．not merely his job in the lab was taken away

D．not just was taken away his job in the lab

单项选择题

有关需要系数的下列说法正确的是( )。

A．反复短时工作，或者连续运行但负荷不稳定的用电设备，需要系数相对较大

B．连续稳定运行的用电设备，需要系数相对较大

C．连续稳定运行的用电设备，需要系数相对较小

D．反复短时工作，或者连续运行但负荷不稳定的用电设备，需要系数相对较小