Psychologist: Research has shown that a weakened immune system increases vulnerability to cancer. So, cancer-patient support groups, though derided by those who believe that disease is a purely biochemical phenomenon, may indeed have genuine therapeutic value, as it is clear that participation in such groups reduces participants' stress levels. Which one of the following is an assumption required by the psychologist's argument? ['Disease is not a biochemical phenomenon at all.', 'Stress can weaken the immune system.', "Discussing one's condition eliminates the stress of being in that condition.", 'Cancer patients can learn to function well under extreme stress.']