Mr. Mann's class conducted an experiment about plant response to the presence of water. They first planted pea seeds in sand. In the beginning, they watered the seeds. Once the seeds began to grow, they reduced the amount of water they gave the plants. Instead, they placed a porous container of water close to the growing plant to see if the plant's roots would grow towards the cup of water. In science language, this plant response is called hydrotropism. An illustration of the experimental design can be seen here. Some of the students also wanted to see if the plant would grow toward the window if they withheld light. They kept their container, with the porous cup, in a dark corner, but close to a window.

What comment can you make about the second group's independent or manipulated variable?

A. Their manipulated variable is plant response to water and light

B. The amount of water and light would be their independent variables

C. Their manipulated variable is the amount of light the plant receives

D. They can only change one variable at a time, either water or light, not both.