Amanda is conducting an experiment on heavy metal uptake in hemp plants (they are believed to remediate soils from heavy metals and she is curious if the heavy metals are accumulated in any specific part of the plant). She is growing plants in containers hydroponically. Along with plant nutrients for fertilizer, she is adding different concentrations of heavy metals to the solution the plants are growing in. She also has a container with just water and fertilizer (no heavy metals added during the growing season). The plants grown in just water and fertilizer would be an example of which of the following:
a. Dependent variable
b. Independent variable
c. Confounding variable
d. Control