单项选择题

We（）on it for many hours but we have not yet reached any conclusion.

A、 work
B、 have been worked
C、 have been working
D、 are working

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单项选择题
（）, such problems concerning environment protection are not as easy to solve as they look.

A、 Speak generally
B、 To speak generally
C、 Generally speaking
D、 General speaking
单项选择题
My boss asked me to answer the phone, to take all messages, and（）some letters.

A、 to type
B、 typing
C、 type
D、 typewrite
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