A.自上而下開(kāi)挖 B.上下結(jié)合開(kāi)挖 C.自下而上開(kāi)挖 D.分期分段開(kāi)挖
A.10~25 B.20~30 C.20~25 D.25~30
A.6 B.8 C.10 D.12