什么是干熄焦？
参考答案：采用惰性气体（通常用氮气）熄灭赤热焦炭的熄焦方法。
A milliner bought a lot of hats, of which were brown. The milliner sold of the hats including of the brown hats. What fraction of the unsold hats were brown ？（）
A.A
B.B
C.C
D.D
E.E
of which were brown. The milliner sold 
of the hats including 
of the brown hats. What fraction of the unsold hats were brown ？（）