Three students each added equal volumes of pond water to four beakers and placed each beaker in a different water bath. each student maintained the water baths at temperatures shown in the data table below. the students then added an equal number of water fleas to each of the four beakers. after one hour, the students used microscopes to determine the average heart rate of the water fleas. the procedure was repeated for a total of three trials at each temperature. the results of the investigation are summarized in the data table below. which component of the experiment above is the independent variable? a. average water flea heart rate b. number of fleas per beaker c. number of trials d. water temperature