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
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.
单项选择题
The new idea the passage presents us is about ___________.
A．designing a new type of heat exchanger
B．storing winter cold for use in summer
C．establishing an underground air-conditional system
D．creating an efficient and economical air-conditioning system
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