主动红外探测器的两种分类方法依据是（）。
A.光束和距离
B.波长和频率
C.光束和频率
D.距离和频率
参考答案：C
从月经初潮至生殖器官逐渐发育成熟的时期称为
A．新生儿期B．幼年期C．青春期D．性成熟期E．更年期
The number of people invited _________ fifty, but a number of them _________ absent for different reasons.
A．were; was
B．was; was
C．was; were
D．were; were
