单项选择题

Making energy use completely harmless to the environment（）very difficult and usually economically expensive.

A．is
B．are
C．have been
D．shall be

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热门试题

单项选择题

One responsibility of a firefighter is to（）.

A．install smoke detectors in residents homes
B．check if smoke detectors are properly installed
C．develop fire safety programs for schools
D．speak to residents about how to prevent fires

单项选择题

Compared with homes without smoke detectors homes with them give their owners a 50% better chance of（）.

A．surviving a fire
B．preventing a fire
C．detecting a hidden fire
D．not getting injured in a fire