普通晶体三极管的放大作用依赖于（）.
A.电流控制作用
B.电压控制作用
C.电场控制作用
D.磁场控制作用
参考答案：A
根据句意及首字母提示填空。
1. You may call the police when you are in t____.
2. Beijing is in the n____ of China.
3. The headmaster called on (号召) the students to r____ money for the poor boy.
4. In China, when you see the yellow lights, please s____ down.
5. I want to book a s____ room with two single beds.
三相交流发电机的三个始端在空间分布上必须相差（）。
A.60o
B.90o
C.120o
D.150o
