单项选择题
A.先开挖后支护 B.自上而下分层开挖,分层支护 C.不得掏底开挖 D.不得上下重叠开挖
A.150m;100m B.110m;90m C.100m;80m D.90m;70m
A.150m B.200m C.250m D.300m
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