单项选择题

When I try to understand () that prevents so many Americans from being so happy as one might expect, it seems to me that there are two causes.

A. why it does
B. what it does
C. what it is
D. why it is

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单项选择题
I had intended () him while he was living at Aberdeen.

A. to visit
B. visiting
C. to have visited
D. having visited
单项选择题
Anesthetics are used()insensitivity to pain during surgical operations.

A. the cause
B. to cause
C. cause of
D. causing
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