The Footprints at Laetoli Were Remarkably Well-Preserved Because

The footprints at Laetoli were remarkably well-preserved because:

A) they were made in an ash slurry that quickly hardened and were then buried by volcanic ash soon after they formed.
B) they were slowly buried by volcanic ash after sitting in the open air for a long period of time.
C) they are very young; their preservation is thus solely a function of their age.
D) they were never buried; the Laetolil Beds that preserve the footprints are the youngest of a series of stratigraphic units.