Experiments are commonly performed in order to determine the effect of one variable upon another variable. Those conducting the study will make changes in one of the variables, called the independent variable. They do so in order to determine what effect such changes have on the other variable, known as the dependent variable. 
 

Most science experiments are about determining the relationship between quantities. In this case, the experimenters wish to determine the relationship between fertilizer brand and growth rate. The purpose could be reworded as "to determine the effect that changing the fertilizer brand has upon the growth rate." This implies that the fertilizer brand will be changed by the experimenters. And as a result of those changes, the growth rate will change since the growth rate values will depend upon the fertilizer brand.

As mentioned in the Fundamentals section (above), the independent variable is the variable that the experimenters are changing. The values of the dependent variable depend upon these changes. So re-read the first paragraph and then make your decision about the answers.
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