单项选择题
A marine sextant has the index arm set at zero and the reflected image of the horizon forms a continuous line with the actual image. When the sextant is rotated about the line of sight the images separate. The sextant has（）.
A.error of perpendicularity B.side error C.prismatic error D.centering error
单项选择题 A large group of stars revolving around a center is known as a（）.
单项选择题 A group of stars which appear close together and form a striking configuration such as a person or animal is a（）.
单项选择题 A first magnitude star is（）.
