单项选择题

It can be inferred from this passage that the new system ___________.

A．has been put into practical use
B．is still on the planning table
C．is still in the experimental stage
D．needs to be improved
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单项选择题
One of the technological problems that will have to be solved in developing the new system is ___________.
A．how to make enough ice underground
B．how to install the whole device underground
C．how to design the proper heat exchanger pipes for the system
D．how to make the tank so as to keep the temperature
单项选择题
If the idea comes true, people ___________.
A．will enjoy the most efficient and economical air-conditioning device
B．can store up a large amount of winter cold and use it anytime
C．can make full use of winter cold to preserve food in summer
D．may use a bit of winter's chill to help them survive a hot summer
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