The phrase "brought on" (Line 1, Para 4. ) most probably means _________.

A．90% of animals and plants went extinct 250 million years ago
B．dinosaurs died off as a result of meteor impacts
C．the "Great Dying" has much to do with volcanic activity
D．global warming played a minor part in the "Great Dying"

A．the atmosphere changed dramatically
B．oxygen levels on the earth became lower
C．the sea levels became lower greatly
D．the sediment gave off enormous carbon and methane