单项选择题
A variable star is one that（）.
A.exhibits a change in magnitude B.has a changing declination C.is increasing in SHA D.is also known as a red giant
单项选择题 A star that suddenly becomes several magnitudes brighter and then gradually fades is a（）.
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（）.
