The milk of many mammals contains cannabinoids, substances that are known to stimulate certain receptors in the brain. To investigate the function of cannabinoids, researchers injected newborn mice with a chemical that is known to block cannabinoids from reaching their receptors in the brain. The injected mice showed far less interest in feeding than normal newborn mice do. Ttherefore, cannabinoids probably function to stimulate the appetite. Which of the following is an assumption on which the argument depends? ["Newborn mice do not normally ingest any substance other than their mothers' milk.", 'The chemical that blocks cannabinoids from stimulating their brain receptors does not independently inhibit the appetite.', 'The mothers of newborn mice do not normally make any effort to encourage their babies to feed.', "Cannabinoids are the only substances in mammals' milk that stimulate the appetite."]