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
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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
单项选择题
What is the advantage of the new air-conditioning system
A．The cost of the air-conditioning device is very low.
B．The working principle of the system is simple.
C．Minimal amount of energy is needed.
D．Only natural resources can be made use of.
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