Of the options given here, B is the best answer.

When conducting an experiment of this sort, what you are trying to do is to find out whether the independent variable has an effect on the dependent variable. For example, you might want to see whether eating a good breakfast...

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Of the options given here, B is the best answer.

When conducting an experiment of this sort, what you are trying to do is to find out whether the independent variable has an effect on the dependent variable. For example, you might want to see whether eating a good breakfast (independent variable) before a test changes students’ test scores (dependent variable). As another example, you mght want to test whether putting a certain kind of fertilizer (independent variable) on a garden patch increases the yield of tomatoes (dependent variable).

In each case, though, you need a control group. You need some students to take the test without eating a good breakfast. You need some tomatoes that are not fertilized with the new fertilizer. If you do not have these control groups, you do not know if changing the independent variable made any difference.

So, B is correct. In the experimental group, you change the chosen variable (the breakfast or the fertilizer) in a known way. Then you do not change that variable in the control group. Finally, you compare the two groups to see if the independent variable made a difference.