简述浮罩式沼气池优缺点。
参考答案：
优点有：压力低且稳定、保温效果好、沼气池可满装、产气效率高；
缺点有：建池成本比较高、占地面积大、出料存在一定困难。
—_____ do you usually go to school?
—_____ about 7:00. [ ]
A. What time; In
B. When; At
C. Where; At
D. How long; At
