When children ____difficulties on their own, they develop new skills and a strong belief in their abilities.
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A friend of mine has decided to give up his well-paid job because the position is too ____and he has no time for his family.
It takes a(n) ____amount of time and energy to achieve proficiency (精通) in a foreign language.
交易成本就是在一定的社会关系中的关系成本（）。
A. 人与自然
B. 人与人
C. 人与商品
D. 人与环境
交易成本包括达成一笔交易所要花费的以外的所有成本（）。
A. 货币成本
B. 时间成本
C. 交易对象成本
D. 机会成本
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