流速仪法测流时，每一测点测速历时不宜少于（），以控制不稳定流速在时间上的变化。

A.v_cp=（v_0.2+v_0.6+v_0.8）/3
B.v_cp=（v_0.2+v_0.4+v_0.8）/3
C.v_cp=（2v_0.2+v_0.6+v_0.8）/4
D.v_cp=（v_0.2+2v_0.4+v_0.8）/4

A.0.3m∕s
B.0.5m∕s
C.0.6m∕s
D.0.7m∕s

A.2cm
B.5cm
C.8cm
D.10cm