单项选择题

Carbon dioxide ______ from burning fuels is the most common of the so-called greenhouse gases.

A．producing
B．having been produced
C．to be produced
D．produced
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单项选择题
Hello! I m calling to cancel the appointment I ______ with Dr. Smith.
A．am making
B．have made
C．will make
D．had made
单项选择题
Every camera we sell comes with a two-year ______.
A．guarantee
B．safety
C．confirmation
D．conservation
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