The student performed an experiment to test his hypothesis about sunlight and plant height using two plants. he kept everything the same between two tomato plants except he exposed them to different amounts of sunlight. he also used a third plant as a control group, or a group in which no variables are manipulated to confirm the results. roberto critiqued the work. which problem did roberto most likely identify?

a.) the sample size is too small.
b.) too many variables are manipulated.
c.) it is not possible to keep the conditions the same.
d.) controls should only be used in studies of animals.