In trying to accumulate knowledge about the universe and to find common factors which underline and account for the facts that he knows,the scientist often chooses the method of the“controlled experiment.”If he wants to find out the effect of light（ ）growing plants,he takes many plants，as alike as possible.

In trying to accumulate knowledge about the universe and to find common factors which underline and account for the facts that he knows,the scientist often chooses the method of the“controlled experiment.”If he wants to find out the effect of light（ ）growing plants,he takes many plants，as alike as possible.