In an investigation designed to determine the effect of the amount of water on plant growth, two groups of equal-sized bean plants of the same species were grown under identical conditions, except for the amount of wafer they were given. One group was watered with 200 milliliters of water once a day, while the other group was watered with 400 milliliters of wafer once a day. After several days, the heights of the plants were measured. It was determined that the plants watered with 400 milliliters of water once a day showed more growth. The independent variable in this investigation is the

A. type of bean plants used in the experiment
B. amount of water given to the plants each day
C. type of soil the bean plants were growing in
D. group of bean plants watered with 200 mL of water