The correct answer is D. All of the above.
The independent variable is the variable that is manipulated by the researcher in an experiment. It is the variable that is changed to see how it affects the dependent variable. The dependent variable is the variable that is measured in an experiment. It is the variable that is expected to change as a result of the changes in the independent variable.
The independent variable is also known as the treatment variable, the experimental variable, or the manipulated variable. This is because the researcher is treating the independent variable in a certain way (manipulating it) to see how it affects the dependent variable.
For example, in an experiment to test the effect of a new drug on blood pressure, the independent variable would be the drug. The researcher would give the drug to some participants and a placebo to other participants. The dependent variable would be blood pressure. The researcher would measure the blood pressure of both groups of participants before and after they take the drug. The researcher would then compare the blood pressure of the two groups to see if there is a difference.
If the researcher finds that the blood pressure of the group that took the drug is lower than the blood pressure of the group that took the placebo, then the researcher can conclude that the drug is effective in lowering blood pressure.