A scientist wants to collect data about the effects of gravity on the growth of soybeans. To test her hypothesis that the soybeans grow better in a zero gravity setting, she separates the soybeans into two groups. The first group is grown in typical still be growing conditions in the Green House on Earth, and the second group is in a zero gravity, yet otherwise identical greenhouse in a space station. Which of the following is the best description of the research design for the study?