3.17 Calcium and bones. Calcium is important for the growth of bone for children. In a study designed to understand how calcium is processed by the body, 40 young girls attended a summer camp where they were fed a controlled diet. The camp ran for two 3-week periods. For one period, the diet included a low amount of calcium. For the other period, there was a high amount of calcium in the diet. The researchers recorded the amount of calcium retained in the body for each girl. Explain why this study is an experiment and identify the experimental units, the treatments, and the response variable. Describe the factor and its levels.