Question: The chemical engineer added a chemical to a fast-burning compound that changed the oxygen consumption once the reaction started. He did a pretest to measure the oxygen consumption index; then he added the chemical and recorded the posttest oxygen index. The pretest values are 120, 139, 122, 120, 124, 120, 120, 125, 122, 123, 126, and 138. The posttest values are 121, 140, 123, 121, 125, 122, 121, 126, 123, 124, 127, and 139. Write the null and alternative hypotheses, select the correct test, and look for differences between the pretest and posttest oxygen index values.