单项选择题

I didn’t manage to work out the problem _______the teacher had explained how.

A．until　　
B．unless　　　　
C．when 　　　　
D．before
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单项选择题
Man cannot prevent the world from being polluted because_______.
A．the population of the world is increasing faster
B．people use too many man-made materials
C．we have more industry 　　
D．we are producing more cars,trucks and buses
单项选择题
The underlined word guaranteed in the passage. Probably means ______.
A．in our charge　　
B．of a little problem　　
C．difficulty 　
D．quite sure
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