Which of the following statements is true？
A. People don't change positions when they are sleeping.
B. A person who sleeps on his stomach is very stubborn and ambitious.
C. A person who sleeps on his side is always anxious and depressed.
D. A person who sleeps curled up is not very sociable (随和的，好交往的).
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经济学家克拉克是主导产业理论研究的先驱者。()
A. 正确
B. 错误
Science museums encourage visitors to touch, listen and operate in order to ______.
A. find out scientific rules for themselves
B. make them feel relaxed
C. make them interested in science
D. all of the above
在纵向关系中，上下游企业的双重加价不会造成效率损失。()
A. 正确
B. 错误
搭配销售是一种纵向价格约束。()
A. 正确
B. 错误
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