In science the meaning of the word "explain" suffers with civilization's every step in search of reality.Science can not really explain electricity,magnetism,and gravition;their effects can be measured and predicted,but of their nature no more is known to the modern scientist than to Thales who first looked into the nature of the electrification of physicists reject the notion that man can ever discover what these mysterious forces "really" are. Electricity, Bertrand Russell says,is not a thing,like St.Paul's Cathedral;it is a way in which things behave.When we have told how things behave when thry are electrified,and under what circumstances they are electrified,we have told all there is to tell.Until recently scientists would have disapproved of such an idea.Aristotle,for example,whose natural science dominated Western thought for two thousand years,believed that man could arrive at an understanding of reality by reasoning from self-evident principle.He felt,for example,that it is a self-evident principle that everything in the universe has its proper place,hence one can deduce that objects fall to the groud because that's where they belong,and smoke goes up because that's where it belongs.The goal of Aristotelian science was to explain why things happen.Modern science was born when Galileo began trying to explain how things happen and thus originated the method of controlled experiment which now forms the basis of scientific investigation.
　　
16. The aim of controlled scientific experiments is _____.
[A] to explain why things happen
[B] to expalin how things happen
[C] to describe self-evident principles
[D] to support Aristelian science

17. What principles most influenced scientific thought for two thoudsand years?
[A] The speculations of Thales.
[B] The forces of electricity,magnetism and gravity.
[C] Aristotle's natural science.
[D] Galileo's discoveries.

18. Bertrand Russell's notion about electricity is _____.
[A] disapproved of by most modern scientists
[B] in agreement with Aristotle's theory of self-evident principle
[C] in agreement with scientific investigation directed toward "how" things happen
[D] in agreement with scientific investigation directed toward "why" things happen

19. The passage says that until recently scientific _____.
[A] that there are mysterious forces in the universe
[B] that man cannot discover what forces "really" are
[C] that there are self-evident principle
[D] that we can discover why things behave as they do

20. Modern science came into being _____.
[A] when the method of controlled experiment was first introduced
[B] when Galileo succeeded in explaining how things happen
[C] when Aristelian scientists tried to explain why things happen
[D] when scientists were able to acquire an understanding of reality by reasoning