Alice is performing an experiment to determine which mouthwash kills the most bacteria. she is testing three different mouthwashes. she uses four pieces of filter paper. each piece of filter paper was 3 cm in diameter. three of the pieces were soaked in each mouthwash for 3 minutes each. the fourth piece was soaked in plain water for 3 minutes as well. each piece of filter paper was placed on bacterial agar plate and covered. the agar plate was placed in a dark area and left alone for seven days. after the seven days was up, alice measured the diameter of the bacterial rings around each piece of filter paper.
the statement that best describes alice's experiment is:
a) the size of the filter paper is serving as a control.
b) the experiment should test only one type of mouthwash.
c) the experiment is well designed and has a valid control.
d) the experiment has too many variables that are being manipulated.