Geographic range is the maximum distance at which a light may be seen under（）.

A.existing visibility conditions，limited only by the curvature of the Earth

B.perfect visibility conditions，limited only by the curvature of the Earth

C.existing visibility conditions，limited only by the intensity of the light

D.perfect visibility conditions，limited only by interference from background lighting

飞行高度层是与一个特定的气压基准（）有关的大气等压面。

A.QFE

B.QNH

C.1013.2hPa

D.各地区规定的气压基准