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    There are fifty students in our class. There are twenty-four boys and twenty-six girls. Some students live near the school, and some others live far from school. About half of the students usually come to school by bike. They often get to school at a quarter to seven. About fifteen students often come to school by bus. They often get to school very early, too. Another ten students come to school on foot. Their homes are near the school, but they are often late for school, because they get up late.

1985年5月，中共中央作出《关于教育体制改革的决定》，明确提出教育体制改革的根本目的是提高民族素质，多出人才，出好人才，贯彻教育为社会主义建设服务，教育与生产劳动相结合，德智体全面发展的方针。此次教育改革（    ）

东汉末年，关中地区人口从240万减少到50万，扬州从320万增加到433万，荆州（今湖北境内）从359万增加到620万．导致这一时期南北人口变化的主要因素是（　　）