In general，the most effective period for observing stars and planets occurs during the darker limit of（）.<br>A:sunset<br>B:civil twilight<br>C:nautical twilight<br>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

出自：海船船员证

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