某城市自来水收费实行阶梯水价,收费标准如下表所示,某用户5月份交水费51元,收所

题型:填空题

问题:

某城市自来水收费实行阶梯水价,收费标准如下表所示,某用户5月份交水费51元,收所用水为____________.
月用水量
不超过12吨的的部分
超过12吨的部分
超过18吨的部分
收费标准
2元/吨
2.5元/吨
3元/吨
考点:一元一次方程的定义一元一次方程的解法一元一次方程中的待定系数一元一次方程的应用
题型:填空题
设命题P:函数f(x)=x+
a
x
(a>0)在区间(1,2)上单调递增;命题Q:不等式|x-1|-|x+2|<4a对任意x∈R都成立.若“P或Q”是真命题,“P且Q”是假命题,则实数a的取值范围是(  )
A.
3
4
<a≤1
B.
3
4
≤a<1
C.0<a≤
3
4
或a>1
D.0<a<
3
4
或a≥1
题型:填空题

下列能源产品中,属于一次能源的是()。

A.柴油

B.石油

C.焦炭

D.煤油

题型:填空题

在汉代察举制中,皇帝根据需要临时指定选士标准和名目的科目叫做( )。

A.常科

B.制科

C.射科

D.特科

题型:填空题

If there is one thing scientists have to hear, it is that the game is over. Raised on the belief of an endless voyage of discovery, they recoil from the suggestion that most of the best things have already been located. If they have, today’s scientists can hope to contribute no more than a few grace notes to the symphony of science.

A book to be published in Britain this week, The End of Science, argues persuasively that this is the case. Its author, John Horgan, is a senior writer for Scientific American magazine, who has interviewed many of today’s leading scientists and science philosophers. The shock of realizing that science might be over came to him, he says, when he was talking to Oxford mathematician and physicist Sir Roger Penrose.

The End of Science provoked a wave of denunciation in the United States last year. "The reaction has been one of complete shock and disbelief, "Mr. Horgan says.

The real question is whether any remaining unsolved problems, of which there are plenty, lend themselves to universal solutions. If they do not, then the focus of scientific discovery is already narrowing. Since the triumphs of the 1960s—the genetic code, plate tectonics, and the microwave background radiation that went a long way towards proving the Big Bang—genuine scientific revolutions have been scarce. More scientists are now alive, spending more money on research, that ever. Yet most of the great discoveries of the 19th and 20th centuries were made before the appearance of state sponsorship, when the scientific enterprise was a fraction of its present size.

Were the scientists who made these discoveries brighter than today’s That seems unlikely. A far more reasonable explanation is that fundamental science has already entered a period of diminished returns. "Look, don’t get me wrong," says Mr Horgan. "There are lots of important things still to study, and applied science and engineering can go on for ever. I hope we get a cure for cancer, and for mental disease, though there are few real signs of progress.

There have not been many genuine scientific revolutions in the past few decades because()

A. there have been decreased returns in the research of fundamental science

B. there are too many important things for scientists to study

C. applied science and engineering take up too much time and energy

D. today’s scientists are not as intelligent as those in the past

题型:填空题

蒽和咔唑提取分离的方法分硫酸法、溶剂法、()三种。

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