单项选择题
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
单项选择题 Elongation becomes zero at（）.
单项选择题 During one synodic rotation，a body makes one complete turn relative to the（）.
单项选择题 Diurnal aberration is due to（）.
