炼焦炉的发展大体可分四大阶段，即成堆炼焦与窑式、倒焰炉、废热式炼炉及现代蓄热式焦炉。
参考答案：对
— What are you going to do this weekend?
— I’m not sure ______. Maybe I am going swimming.
A．also
B．too
C．ever
D．yet
原始地球表面没有生命，但有相当量的游离氧存在。 [ ]
