北京人使用的劳动工具主要是（  ）

②滴入氢氧化钠溶液，三支试管中均没有出现预料中的蓝色絮状沉淀：小刚的试管中是黑色浑浊；小强向试管中是蓝色溶液；小柯的试管中虽然出现过少量蓝色沉淀但又马上变成了黑色沉淀．于是三位同学开始探究实验失败的原因，经过一番讨论和观察他们认为与反应物的量和条件有关：

请和你的同学一起讨论讨论，找出答案。

    Tom, Sam, Jim and Jack live in a building of four floors. They live on different floors. They have different jobs―an engineer, a worker, a teacher and a doctor. Tom lives above Sam, but below(在下面) Jim. Jack lives on the third floor. The doctor lives above the teacher, but below the worker. The engineer lives on the ground floor. What are their jobs? Which floor does each of them live on?