X线透视可见两侧膈下线条状或新月状透光影，多见于（）。E．
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【C15】
A. couple
B. spouse
C. pair
D. twin
A variable whose value is determined by the model of which it is a part is termed______.
A. constant
B. endogenous
C. exogenous
D. independent
If lighthouses are public goods, then free markets would______.
A. create a lighthouse monopoly
B. provide the optimal number of lighthouses
C. provide more than the optimal number of lighthouses
D. provide less than the optimal number of lighthouses
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