单项选择题
After Venus passes the point of greatest elongation east in its orbit，the first position in which the elongation will be zero is（）.
A.superior conjunction B.inferior conjunction C.opposition D.None of the above; the elongation will never be zero
单项选择题 A variable star is one that（）.
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（）.
