Mass extinctions occurred at, or near the end, of which three periods?
a. _____ Cambrian, Ordovician, Permian; b. _____ Cambrian, Silurian, Devonian; c. _____ Ordovician, Devonian, Permian; d. _____ Silurian, Devonian, Permian; e. _____ Cambrian, Devonian, Permian.
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