The level оf lymphоcytes (white blоod cells) in the blood is аssociаted with susceptibility to diseаse--lower levels indicate greater susceptibility. In a study on the effects of stress on well-being, researchers recorded the lymphocyte counts for men who were going throug a period of extreme emotional stress (the death of a spouse). The researchers measured the lymphocyte counts for each man one week after the death, and the measured each man's levels of lymphocytes two weeks later. What test will tell the resarchers whether there was a signfiicant change in lymphocyte counts?