--Have you returned the book to the library _______? --Yes, I have ____ returned it.

A．yet, yet

B．already, already

C．already, yet

D．yet, already

斯科特系统氮气置换时，采用的方法为（）。

A、用循环风机置换系统氧

B、用氮气向火炬赶氧

C、用氮气赶氧至焚烧炉

D、将氮气就地放空置换