四缸四冲程柴油机曲轴每转一周，有( )个气缸做功。
A．1
B．2
C．3
D．4
参考答案：B
The mayor ordered that the food and tents ________ sent to the people in the earthquake as soon as possible.
A．were
B．be
C．would be
D．had been
从灯丝正面发射出的电子所形成的焦点称为（）
A.实际焦点
B.有效焦点
C.等效焦点
D.主焦点
E.副焦点
