The passage below has gaps numbered 11 to 20. Immediately following each gap, four options are provided. Choose the most appropriate option for each gap.

  It is the business of the scientist to accumulate knowledge about the universe and all that is in it and to find if he is able, common……11…..[A. experiment B. instruments C. approaches D. factors] which underlie and account for the facts that he knows. He chooses when he can, the method of the controlled…….12…..[A. respondent B. experiment C. system D. data]. If he wants to find out the effect of light on growing plants, he takes many plants, as alike as possible. Some he stands in the sun, some in the shade, some in the dark, all the time keeping all other.....13.....[A. studies B. procedures C. objects D. conditions]  (temperature, moisture, nourishments) the same. In this way, by keeping other variables……..14……[A. constant B. good C. dark D. natural] and by varying the light only, the effect of light on plants can be clearly seen. This …….15…..[A. research B. method C. tool D. rationale] of using ‘controls’ can be applied to a variety of situations and can be used to find the answers to questions as widely different as ‘must moisture be present if…….16….[A. an alloy B. gold C. bar D. iron] is to rust?’ and which variety of beans give the greatest yield in one……17…..[A. climate B. period C. season D. weather]?

  In the course of his……18…..[A. finding B. queries C. experiment D. inquiries], the scientist may find what he thinks is one common explanation for an increasing number of facts the explanation, if it seems consistently to fit the various facts, is called…….19……[A. an antithesis B. a principle C. a thesis D. a hypothesis]. If this continues to stand the test of numerous experiments and remains unshaken, it becomes a……20…..[A. deduction B. law C. notion D. thesis].