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问题 解答题

自然数由合数和质数组成．______．（判断对错）

答案

根据质数与合数的意义可知，

最小的质数为2，最小合数是4，而自然数包括0、1等整数，

所以自然数由合数和质数组成说法错误．

故答案为：错误．

选择题

.著名画家范曾先生在《自述》中谈到自己的成功时说：“在艰苦颠蹶之中，我没有沉沦，我追逐着光明和希望。”由此可见，一个人要想取得成功，必须

A．要处在艰苦的环境中

B．既要注重量的积累，又要善于促成质变

C．尽力回避不利的条件

D．既要看到前途的光明，又要勇于克服困难
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单项选择题

The level of lead contamination in United States rivers declined between 1975 and 1985. Federal regulations requiring a drop in industrial discharges of lead went into effect in 1975, but the major cause of the decline was a 75 percent drop in the use of leaded gasoline between 1975 and 1985.
Which of the following, if true, best supports the claim that the major cause of the decline in the level of lead contamination in United States rives was the decline in the use of leaded gasoline

A．The level of lead contamination in United States rivers fell sharply in both 1975 and 1983.

B．Most of the decline in industrial discharges of lead occurred before 1976, but the largest decline in the level of river contamination occurred between 1980 and 1985.

C．Levels of lead contamination in rivers fell sharply in 1975-1976 and rose very slightly over the next nine years.

D．Levels of lead contamination rose in those rivers where there was reduced river flow due to drought.

E．(E) Although the use of leaded gasoline declined 75 percent between 1975 and 1985, 80 percent of the decline took place in 1985.
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