单项选择题
A star that suddenly becomes several magnitudes brighter and then gradually fades is a（）.
A.double star B.variable star C.nova D.nebula
单项选择题 A star is observed at lower transit. The line of position derived from this sight is（）.
单项选择题 A sidereal day is shorter than a solar day. This difference is due to（）.
单项选择题 A sidereal day is approximately how much shorter than a solar day?（）
