A.2m B.3m C.4m D.5m
A.45min B.1h C.2h D.2.5h
A.上弦節(jié)點(diǎn)間 B.上弦節(jié)點(diǎn)處 C.下弦節(jié)點(diǎn)間 D.下弦節(jié)點(diǎn)處