What appears to be the "impact history" of cratering on the Moon?
A. short periods of heavy bombardment alternating with long periods of light bombardment throughout the Moon's life
B. heaviest bombardment when the Moon first formed, gradually decreasing (except for increased activity about four billion years ago) to light bombardment today
C. more or less constant bombardment from the earliest times to the present
D. an early period of heavy bombardment followed by a decrease in intensity (except for a spike around four billion years ago) and then very light bombardment to the present