If the primal LP problem has optimal solutions, then（）
A. The dual problem is bounded
B. The feasible region of the dual problem is nonempty
C. The dual problem has an optimal basic feasible solution
D. The dual problem is non-degenerate
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Which of the following assertions for the feasible region of a standard LP problem is incorrect（）
A. The feasible region is convex
B. The feasible region might not have vertex
C. The feasible region might have infinitely many vertices
D. The feasible region might be unbounded
点的水平投影到OX轴的距离等于点的侧面投影到的距离（）
A. OY轴
B. OZ轴
Which of these network devices primarily functions at the OSI Network layer（layer 3）（）
A. Switch
B. Gateway
C. Router
D. All of the above
Two parallel school systems in the UK are______（）
A. national and dependent
B. state and dependent
C. state and independent
D. regional and state
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