A researcher plans to study the effect of temperature on the rate of metamorphosis from tadpole to frog. The researcher sets up two tanks with one tadpole in each. Both tanks are filled with water from a nearby pond. One tank is kept at 15°C and the other at 25°C. How should this experiment be improved? A: use tap water in one tank and pond water in the other
B: keep both tadpoles I. The same tank and vary its temperature
C: use tadpoles of 2 different species
D: have may tadpoles in each tank