晶闸管直流斩波器的换流方式有（）和电流换流两种。
参考答案：电压换流
Jay Chow's __________is so beautiful that most young people like his songs.[ ]
A. sound
B. whisper
C. noise
D. voice
蛋白质生物合成的抑制剂是
A．卡那霉素
B．甲氨蝶呤
C．利福霉素
D．氮杂丝氨酸
