Mr. Mann's class conducted an experiment about plant response to the presence of water. They first planted pea seeds in the sand. To start with, they watered the seeds. Once the seeds began to grow, they reduced the 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. What is the manipulated variable in this experiment?
A) plant root growth
B) the sand used to grow the seeds
C) limited watering, replaced by the cup of water
D) the water used to germinate the seeds and get them growing