单项选择题

When I try to understand （）that prevents so many Americans from being as 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
Many students will watch TV only to （） time during the summer vacation.

A．spend
B．waste
C．enjoy
D．kill
单项选择题
Many companies are seeking to exploit and develop the rich natural（）in western regions.

A．sources
B．resources
C．materials
D．power
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