What is mentioned as a major characteristic of the one-room school system in the
A. Some children have to be left back.
B. Teachers are always busy.
C. Pupils have more freedom.
D. Learning is not limited to one grade level at a time.
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请选择（7）处的最佳答案．
A. gathered
B. picked up
C. selected
D. chosen
A．orB．untilC．butD．then
A. or
B. until
C. but
D. then
请选择（4）处的最佳答案
A. changing
B. variable
C. deviate
D. diverse
It can be inferred from the passage that white can be produced by
A. mixing different proportions of magenta, green and red
B. mixing equal proportions of green, red and blue
C. mixing different proportions of cyan, yellow and blue
D. mixing equal proportions of black, red and green.
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