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 is implied in the last paragraph that _________.

A．only those lived near sea could survive
B．lack of oxygen led to the extinction of most of the species
C．The living space was too small to hold so many species
D．Global warming is related to human activities

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