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当前学科：连铸高级工
题目：
单选
铸坯的断面裂纹产生与（）有关。
A . 扇型段
B . 二次冷却
C . 扇型段开口度与二次冷却
D . 结晶器
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Once we learn the rules for map reading, they seem so simple: up is north, blue is water, a star stands for a capital. But when and how do we learn to read a map? Research shows that even three-year-olds can master, at least in a basic way, the key map-reading concept(概念): a map is a two-dimensional symbol(二维的符号)for three-dimensional space. Linda Acredolo of the University of California, showed a child a map of a room pointed out on it where a toy was hidden and asked the child to find that toy. Depending on the maps orientation(方位), about half the three-year-olds could successfully do so. Three years may be the age at which the ideas become fully understood. Judy Deloache of the University of Illinois let children watch her hide a small toy in a scale model of a room and asked them to find a large one in a real room. "A three-year-olds can do it, but a two-year-olds has no idea that the model has to do with anything larger, " she says. But even if the basic concept is there early, Lynn Liben and Roger Downs of Pennsylvania State University found that children still have a lot to learn. Second graders believe that north is always at the top of a map because of the hanging maps they have seen. "If you asked them to close their eyes and point in the direction of north, often theyll point straight up," says Lihen. She says that children get map-reading skills gradually, and teachers must begin by using maps of familiar areas: their own classrooms, homes, and neighbourhoods.The first two experiments show that______.A．children have better understanding than grown-upsB．children have special natural ability for map readingC．children are more interested in toys than in mapsD．children can start to learn map-reading early
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