单项选择题

The concentration of carbon dioxide is caused by_______.

A．the heat accumulated in the atmosphere
B．the burning of coal
C．the wide use of renewable energy
D．the destruction of the tropical rain forest
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单项选择题
A．toB．withC．onD．for
A．to
B．with
C．on
D．for
单项选择题
The process by which carbon dioxide and oxygen are transferred does not depend on_______.
A．the presence of nitrogen in the blood
B．the muscles of the thoracic cavity
C．the flow of blood
D．the moisture in the air sac linings
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