干扰电流治疗作用中兴奋交感神经的差频（）
A.100Hz
B.50～100Hz
C.25～50Hz
D.20～60Hz
E.1～10Hz
参考答案：E
With a lot of difficult problems____________ , the manager felt worried all the time.
A．to settle
B．settling
C．settled
D．being settled
