Answer :
The answer is D. For an experiment, there is independent variable and dependent variable. Independent variable is the control variable that needs to change. Dependent variable is the one that need to be observed. So, generally speaking, in one experiment, one variable needs changing and others need to be constant.
D. one or more variables are kept constant.
In a controlled scientific experiment, a scientist alters one variable, called the independent variable. The scientist then studies the effects of this alteration on another variable, called the dependent variable. Any other variables involved the experiment are kept constant.
