A laboratory experiment was done to show the effects of organic waste on the dissolved oxygen (DO) content in water. Five tanks were set up, each containing fresh water and a small amount of single-celled green algae. Specified amounts of organic waste were added to the tanks. The results below show the amount of DO in each tank after a period of one week.
Tank 1 Tank 2 Tank 3 Tank 4 Tank 5
Initial DO 10 ppm 10 ppm 10 ppm 10 ppm 10 ppm Amount of organic Waste added
0 g 10 g 20 g 30 g 40 g
DO after one week
10 ppm 10 ppm 8 ppm 5 ppm 0 ppm
Which of the following would best improve the validity of the experiment?
a. Increasing the amounts of green algae in each of the tanks, with tank 5 having the greatest amount
b. Eliminating tank 1, because the DO stayed the same.
c. Adding different types of waste to each tank and then checking the results.
d. Repeating the experiment several times and comparing the results.