单项选择题

The greenhouse effect resulted hundreds of million years ago when _________.

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

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热门试题

单项选择题

It can be inferred that dinosaurs' low-oxygen adaptation _________.

A．was unique in evolution
B．evolved out of the low-oxygen environment hundreds of million years ago
C．is hard to explain
D．caused their extinction

单项选择题

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

A．cause to happen
B．accelerate
C．diminish
D．sustain