题目

    己知：四点A、B、C、D的位置如图所示，根据下列语句，画出图形．     (1)画直线AD、直线BC相交于点O;     (2)画射线AB． 答案：．(1)画图正确································································································· 2分 结论························································································排纵队时，如果看到前面的一位同学挡住了他前面所有的同学，队就排直了，这可以用________　　　         　来解释，光在_______中的传播速度最快，其速度是_____________m/s．