In an experiment, onion plants of different ages were exposed to various amounts of caffeine to determine the effect of caffeine on cell growth. temperature, amount of water, and light were kept constant. which of these statements about the experiment is definitely true?
a. the results may not be valid because there were two variables.
b. the amount of caffeine used was a control.
c. temperature, water, and light were the only variables.
d. onion plants can be used to model the effects of caffeine on living things.