Since 1969 what has happened to the footprints left by the Apollo 11 astronauts?
A. The astronauts were required to clean up their footprints, so the Moon would remain pristine.
B. They have long since vanished because of fierce Solar winds.
C. Erosion will gradually wear them away, but at a much slower rate than on Earth.
D. They are as they were in 1969 because there is no air or water on the Moon to disturb them.