What is the celestial sphere?

A. The celestial sphere is a representation of how the entire sky looks as seen from Earth.
B. The celestial sphere is a model that shows the true locations in space of the Sun and a few thousand of the nearest stars.
C. The celestial sphere is a model of how the stars are arranged in the sky relative to our Sun, which is in the middle of the sphere.
D. It represents a belief in an Earth-centered universe, and hence is no longer considered to have any use.