集成中频放大器中，常用的集中滤波器有（）、陶瓷滤波器、晶体滤波器、声表面波滤波器等四种。
参考答案：LC带通滤波器
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
简述美国心理学家奥尔波特提出了心理健康的6条标准。
