单项选择题

The barrier was used ______ protection which kept the enemy out of frontier towns.

A. as means of
B. as mean of
C. as a means of
D. as a mean of
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We do not do much ______ with him.
A. commerce
B. businesses
C. business
D. enterprise
单项选择题
Shingles ______ by a virus which attacks the nerves.
A. is causing
B. is caused
C. are caused
D. are causing
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