三极管的三个极是：（）
A、阳极、阴极和地
B、正极、负极和外皮
C、发射极、基极和集电极
参考答案：C
The time and effort he has devoted during the past few years____ trees in that remote area is now considered to be of great value．
A．to plant
B．to planting
C．plant
D．planting
