单项选择题

A．The high temperatures required for its use.
B．The high cost of materials used in its production.
C．The lack of trained environmental engineers.
D．The opposition of automobile manufacturers.
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单项选择题
A．At 10:00.B．At 8:00.C．At 7:30.D．At 8:30.
A．At 10:00.
B．At 8:00.
C．At 7:30.
D．At 8:30.
单项选择题
Mouthwashes are not an effective cure for bad breath mainly because ______.
A．they can’’t mask the bad odor long enough
B．they can’’t get to all the offending bacteria
C．their strong smell mixes with bad breath and makes it worse
D．they can’’t cover the thick layers of mucus
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