单项选择题

In the example cited in the passage, the opportunity cost of studying for 4 hours per day was________.

A．4 hours of watching TV
B．4 hours of classes
C．4 hours of sleep
D．8 hours of sleep
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单项选择题
According to the passage, choices must be made because________.
A．resources are abundant
B．there is too much for us to do
C．there are only 24 waking hours in a day
D．our means are limited, but our wants are unlimited
单项选择题
He tried to cope with the ever-increasing burden of his work, but finally he ________and had to take a complete rest.
A．broke through
B．broke out
C．broke into
D．broke down
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