用臭氧消毒时，一般一级处理出水的臭氧投加量为（）mg/L。
A.10～20
B.20～30
C.30～50
D.40～60
参考答案：C
Tom's grandfather ____ for two years. [ ]
A: died
B: has been dead
C: has been death
D: had died
绘图题：画出三相全控桥＋A和－A相快熔熔断后的输出波形图。
