一光滑圆环固定在竖直平面内，环上套着两个小球A和B（中央有孔），A、B间由细绳连接，它们处于如图所示位置时恰好都能保持静止状态．此情况下，B球与环中心O处于同一水平面上，AB间的细绳呈伸直状态，与水平线成30°夹角．已知B球的质量为1kg，取g=10m/s²，求：

      A poor woodcutter and his wife lived in the forest in a small hut.   1 was so small that there was space only for him and his wife to sleep.
      One cold night, somebody knocked 2 the door. The woodcutter asked his wife  3the door and let the stranger in. She said, “ But there is no space.” The man laughed 4 said, “ Two can sleep well, three can sit.” And then the woman opened the door. The stranger5 to come in, and they all sat telling stories to each other.
     Then came 6 knock. The woodcutter told the stranger, “Friend, open the door.” And the man said, “But there is no space.” The woodcutter said, “If three persons can sit comfortably, four persons can sit a little7 than before.”
The stranger opened the door, and a man entered. Suddenly, a strange knock 8 did not seem to be human's! The woodcutter said, “Open the door. I know who 9.  It is my donkey.”
    Everyone said, “  10is the donkey going to stand?” The man said, “Right now we are sitting, when the donkey comes in, we will all stand.”

铁是由同种 ____组成的纯净物，属于 ____，由铁 ____构成；二氧化碳是由不同 ____组成的纯净物，属于 ____，由二氧化碳 ____构成。