```The passage below has gaps numbered 6 to 10. Four options are provided for each number. 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 ___6___ which underlie and account for the facts that he knows. He chooses, when he can, the method of the controlled common ___7___ 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 ___8___ (temperature, moisture, nourishment) the same. In this way, by keeping other variables ___9___ and by varying the light only, the effect of light on plants can be clearly seen. In the course of his ___10___, The scientist may find what he thinks is one common explanation for an increasing number of facts.```

• A.experiments
• B.instruments
• C.approaches
• D.factors