One seed was planted in each of 5 pots found around the house that contained 500g of "pete`s potting soil." the pots were given the following amounts of distilled water each day for each day of 40 days: pot 1, 50ml; pot 2, 100ml; pot 3, 150ml; pot 4, 200ml; pot 5, 250ml. because pot 3 received the recommended amount of water it was used as a control. the height of each plant was measured at the end of the experiment. what is the independent variable?