Which of the following is not a piece of evidence supporting the idea that an impact caused the mass extinction that occurred about 66 million years ago?
a) Fossilized dinosaur bones contain fragments of rock from the impact.
b) Unusually large abundances of iridium and other rare metals are found in a layer of clay that dates to about 66 million years ago.
c) A large impact crater along the coast of Mexico dates to about 66 million years ago.
d) Grains of quartz formed under high pressure are found in a layer of clay that dates to about 66 million years ago.