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（）.

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（）.