Mia enjoys studying fossils. She learned how fossils called index fossils are found in the rock layers. The index fossils help scientists study rock ages. In what way do index fossils help determine rock ages?
A. Index fossils are widely distributed but were not around long, and so they show up in only a few rock layers.
B. Index fossils are widely distributed and were around for a long time, and so they show up in many rock layers.
C. Index fossils were around only a short time but they show up in many rock layers.
D. Index fossils are widely distributed and are always found in every sedimentary rock layer.