简述半塑式沼气池优点。
参考答案：
1.结构简单，施工方便。
2.投资少，成本低。
3.产气率高。
4.管理方便。
—Could you tell me __________?
—Certainly. In half an hour.
A．when will the train to Beijing leave
B．when the train to Beijing would leave
C．when the train to Beijing left
D．when the train to Beijing leaves
