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also…
b.) What was the critical evolutionary event which led to the tremendous diversity of grazing animals?
c.) What can be said regarding the spread of mammals into various niches of life [air, fresh water, oceans, land]?

Approximately 65 million years ago, a mass extinction occurred on planet Earth. Layers of soil dating back to this time contain large amounts of the radioactive element iridium. This indicates that a large asteroid or comet struck Earth, throwing up huge clouds of debris, which would block much of the sunlight. Food became very scarce. Half of the marine species and nearly all of the dinosaur species went extinct during this time. Soon after the cloud settled, the surviving species of mammals rapidly diversified and filled the niches left empty by the dinosaurs. What evolutionary event was represented by this asteroid collision and the resulting cloud?
a. mutation
b. gene flow
c. genetic drift
d. natural selection
Thanks!