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问题 单项选择题

有关固有不清晰度的以下说法正确的（）

A.在射线照相中的固有不清晰度是由于射线在对胶片感光时，射线触发胶片的乳剂产生了一部分光电子，然后对胶片产生感光所引起的

B.一般来讲射线愈硬，固有不清晰度愈大

C.在射线照相时不宜采用过硬的射线进行照相

D.以上都对

答案

参考答案：D

单项选择题

The United States Food and Drug Administration (FDA) regulates the introduction of new therapeutic agents into the marketplace. Consequently, it plays a critical role in improving health care in the United States. While it is those in the academic and government research communities who engage in the long process of initial discovery and clinical testing of new therapeutic agents, it is the FDA’s role and responsibility to facilitate the transfer of new discoveries from the laboratory to the marketplace. Only after the transfer can important new therapies help patients.
Which one of the following statements can be inferred from the passage

A．The FDA is responsible for ensuring that any therapeutic agent that is marketed is then regulated.

B．Before new therapeutic agents reach the marketplace they do not help patients.

C．The research community is responsible for the excessively long testing period for new drugs, not the FDA.

D．The FDA should work more closely with researchers to ensure that the quality of therapeutic agents is maintained.

E．(E) If a new medical discovery has been transferred from the laboratory to the marketplace, it will help patients.
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多项选择题

一次通话的时间长度服从指数分布Exp(0.49)，则一次通话的平均时长E(X)与标准差σ(X)分别为( )。

A．E(X)=1/0.49

B．E(X)=1/0.7

C．σ(X)=1/0.7

D．σ(X)=0.7

E．σ(X)=1/0.49
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