----How long does it take us to go to your hometown from here?

----It takes us      hour or more to go to my hometown by       train.

A．an; a

B．a ; an

C．an; /

D．a; /

古语道：“人要实，火要虚”．从燃烧的条件看，“火要虚”的实质是（　　）

A．增大可燃物的热值

B．提高空气中氧气的含量

C．提高可燃物的着火点

D．增大可燃物与空气的接触面积