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问题 解答题

在分别写有1～20的20张小卡片中，随机抽出一张卡片，试求以下事件的概率．

A、该卡片上的数学是5的倍数．  B、该卡片上的数字是素数．

答案

卡片上的数学是5的倍数的数字有5，10，15，20共4个，所以P（A）=

4
20
=
1
5
；

卡片上的数字是素数的有2，3，5，7，11，17，19共7个，所以P（B）=

7
20
．

单项选择题

Guitar strings often go "dead"—become less responsive and bright in tone—after a few weeks of intense use. A researcher whose son is a classical guitarist hypothesized that dirt and oil, rather than changes in the material properties of the string, were responsible.
Which of the following investigations is most likely to yield significant information that would help to evaluate the researcher’s hypothesis

A．Determining if a metal alloy is used to make the strings used by classical guitarists.

B．Detennining whether classical guitarists make their strings go dead faster than do folk guitarists.

C．Detennining whether identical lengths of string, of the same gauge, go dead at different rates when strung on various brands of guitars.

D．Determining whether a dead string and a new string produce different qualities of sound.

E．(E) Determining whether smearing various substances on new guitar strings causes them to go dead.
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单项选择题

钢材在低温下，塑性（）。

A.提高

B.下降

C.不变

D.可能提高也可能下降
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