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

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单项选择题
Elongation becomes zero at（）.

A.opposition
B.west quadrature
C.apogee
D.inferior conjunction
单项选择题
During one synodic rotation，a body makes one complete turn relative to the（）.

A.Earth
B.Sun
C.stars
D.vernal equinox
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