A.先開(kāi)挖再支撐 B.開(kāi)槽支撐先撐后挖 C.邊開(kāi)挖邊支撐 D.挖到槽底再支撐
A.0.4 B.0.2 C.0.8 D.0.7
A.20cmB.25cmC.30cmD.35cm