The answer you're looking for is D. Boiling points of various substances. I got this answer because the independent variable is the thing in an experiment that is unaffected by any thing in the experiment, but has to be changed by the tester to find a change in the outcome of the dependent variable. It can't be A because depending on the temperature of the heated container must be different to reach the boiling point of different substances, for this reason, we also find out that D is the answer. I hope that this helps!
