Study 1 Store supervisors studied the variation of earnings per day with average shelf stock of vegetables. They maintained different amounts of shelf stocks of selected vegetables during different weeks. The supervisors observed the corresponding earnings from vegetable sales. Study 2 Store supervisors studied the variation of earnings per day with the average marked price of selected items. They marked the items with different prices during different weeks and observed the corresponding earnings from sales of those items. Two studies conducted in a chain of grocery stores are described in the table. Which statement correctly classifies the two studies?