Why Do Satellites Launched into Low-Earth Orbits Not Remain There

Multiple Choice

Why do satellites launched into low-Earth orbits not remain there indefinitely?

A) they do not have enough speed to fall freely around the Earth
B) they tend to collide with other spacecraft and spacecraft fragments
C) they lose speed due to friction with the upper layers of the Earth's atmosphere
D) they run out of fuel, and without fuel, all satellites, no matter what their orbit is, must fall
E) this is an unsolved problem, but a NASA committee has been assigned to work on it