单项选择题

How has the use of helicopters developed

A．They are taking the place of high-flying jets.
B．They are used for rescue work only.
C．They have been widely used for various purposes.
D．They are now used for commercial purposes.

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单项选择题

What does the author mean when he says we can't turn the clock back (Line 1, Para. 3)

A．It's impossible to slow down the pace of change.
B．The social reality children are facing cannot be changed.
C．Lessons learned from the past should not be forgotten.
D．It's impossible to forget the past.