备用电源断开灯何时能亮()
A.电瓶拆离飞机后
B.备用直流汇流条失电时
C.备用交流汇流条失电时
D.以上都是
参考答案:D
After a whole day’s heavy work ,the old worker returned home ,______________________.
A.hungry and tiring
B.hunger and tired
C.hungry and tired
D.hunger and tiring
吸式焊炬在点火时应先开氧气阀门,再开乙炔气阀门,然后(),调整火焰。熄火时应先关闭乙炔气阀门,再关闭氧气阀门。