渭化气化炉黑水系统的仪表控制采用（）。
A、液位分成调节
B、液位和流量串级调节
C、流量和压力串级调节
参考答案：B
He rushed out of the burning house with the little boy in his arms ____________ the roof fell down and the boy was saved.
A．after
B．as
C．before
D．until
简述说明换相失败的过程。
