20. I got up early this morning, but I ________ so because I was not on duty today.

A．mustn’t have done

B．needn’t have done

C．didn’t need to do

D．can’t have done

对含硫油气层上部的非油气矿藏开采层应下套管封住，套管鞋深度应大于开采层底部深度（）m以上。

A、100

B、300

C、200