I enjoyed the concert last night; they played ______ beautiful music.

A．They are falling out of use in the United States.
B．They tend to be ignored by drivers in a short period of time.
C．They are applicable only on broad roads.
D．They cannot be applied successfully to traffic circles.

A．We have an inexhaustible supply of energy from it.
B．This could bring the world to an energy crisis.
C．The sources of energy on Earth would become unreliable.
D．We find ourselves in difficult position with environmental pollution.