决策支持系统的基本结构是人机交互系统、模型库系统和数据库系统三个子系统的有机结合。
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列车司机在列车运行中，遇有信号显示不明或危及行车和人身安全时，应立即采取（）措施。
If you have to jump in the water when abandoning a vessel，your legs should be（）.
A. spread apart as far as possible
B. held as tightly against your chest as possible
C. in a kneeling position
D. extended straight down and crossed at the ankle
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