What are two ways in which comets are different from other celestial bodies?

a. nucleus is rocky, with dust and ice; often found between mars and jupiter
b. nucleus is a gas ball; usually come from far away from the sun
c. nucleus of dust, rocks, and ice; may have a long, gaseous trail
d. called a meteor as soon as it develops the long gaseous tale; may have an orbit that takes more than 100 years to complete