In general,the most effective period for o
题型:单项选择题
问题:
In general,the most effective period for observing stars and planets occurs during the darker limit of().
A.sunset
B.civil twilight
C.nautical twilight
D.astronomical twilight
In general,the most effective period for observing stars and planets occurs during the darker limit of().
A.sunset
B.civil twilight
C.nautical twilight
D.astronomical twilight