Question

The main reason it's important to only test one variable at a time is so that?

Answer 1

Im almost certain the answer Is under letter A, if not it is D

Answer 2

The main reason as for why it is best and important to test one variable at a time is because B. Scientists can easily tell which variable would have primarily caused different results to have appeared. If many variables are tested, it would be much more difficult to assess and look at the relationship between 2 specific variables and plot data if needed.