Mars's moon, phobos, has an irregular shape and is best known for its many craters, including one gigantic crater. (2) deimos also has a number of craters, but some of its surface looks smoother because some of the craters have been filled in. (4) some scientists believe mars's two moons are asteroids that got caught by mars's gravity.

which of these choices is the best topic sentence for the composition?

a)
phobos and deimos are full of craters.

b)
earth is not the only planet with a moon.

c)
mars has two moons, named phobos and deimos.

d)
planets sometimes pull asteroids with their gravity