Which of the following statements about comets is true?
A. Most comets have a circular orbit that keeps them in the space between Mars and Jupiter.
B. Comets have very elliptical orbits that usually take them far beyond the orbit of Pluto, but also take them closer to the Sun than Earth.
C. Comets rarely fall into an orbit around the Sun. They usually enter the inner solar system only once and then are gone forever.
D. Comets have very elliptical orbits that usually take them closer to the Sun than Earth, but rarely do they get further away than Pluto.