据中国气象网，北京时间2016年1月6日06:08:21小寒。小寒时节（小寒后约15天）多是我国最冷时节，也是雾霾多发时节。小寒时节采用人工降雨方式减轻雾霾，比较适合于（      ）

    In the past black people in South Africa ____(live) a miserable life. They could not vote to choose their ____(leader). They could not get jobs they wanted. The parts of town in which they had to live ____ (decide) by white people. They lived in the ____(poor) areas in the country and could not get jobs they wanted. Nelson Mandela, a modern hero, called black people ____ fight against white people to make black and white people equal. He said “We were put in a position ____we had either to accept we were less important,____ fight the Government. We chose to attack the law. We first broke the law____a way which was ____(peace); when this was not allowed…only then ____we decide to answer violence with violence.”

已知⊙O中最长的弦为16cm，则⊙O的半径为____cm．

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