An independent variable is a. directly changed by the experimenter. b. manipulated by changes to the dependent variable. c. a variable that is kept constant. d. a variable that is used as a control.

In a linear equation or in an experiment there are two types of variables, the independent variable and the dependent variable. The value of the dependent variable is dependent on the changes made by the experimenter in the value of the independent variable. Thus, the answer to this item is letter A.

So we want to know what is the independant variable. When we are making an experiment we need to control the variables in a proces called control of variables. There we keep all of the variables constant and we change the independant variable to see how it is related to the other variables. So the correct answer is a. directly changed by the experimenter.