如图，工人要将一块重900N的建材运到10m高处，装置中每个滑轮重100N，建材上升的速度是0.8m/s．在施工过程中，下列说法正确的是（　　）

有一口水井．在无渗水的情况下，甲抽水机用20小时可将水抽完，乙抽水机用12小时可将水抽完．现在甲、乙两台抽水机同时抽，由于有渗水，结果用9小时才将水抽完．在有渗水的情况下，用甲抽水机单独抽需多少小时抽完？

    One night, Mary heard some big1noise.  “Doh, Doh, …”It2 for a few minutes.  Mary went out of her bedroom to search everywhere.  Her parents were sleeping and there were no 3people in the house.  Mary 4 to her bedroom.
    Mary heard the noise again.  She thought, “Is it a cat?  Mike has a cat.  But cats miaow.  They don't make such noise.”
  “Maybe it's a bird5 around and looking for a place to rest.  No! That doesn't make any noise.”
    Maybe it's the whisper of my parents.  They 6 be7in their room.  But that noise wasn't a whisper at all.”
    Mary went to bed again but she heard the noise again. She was 8 and fell 9 her bed. Her legs were10  and she could not climb up any more.

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