A.1/4~1/6 B.1/5~1/7 C.1/6~1/8 D.1/7~1/9
A.先探后掘 B.邊掘邊探 C.先掘后探 D.掘后再探
A.0.1~0.5m B.0.2~0.6m C.0.3~0.7m D.0.4~0.7m