Why does the instructor mention physical illness?
细节题。根据In the case of physical illness，the boundary lines between normality and pathology are often clearly depictedby medical science，making it easier to diagnose．0n the psychological level，however，we have no ideal model to use as a base of comparison，nothing to help US distinguish mental health from mental disorder．可知医学科学对身体疾病的常态和病理有着清楚的描述，因此，身体疾病较为容易诊断。然而，在心理层面上．目前并没有一套区分心理健康与心理疾病的完善模式。提出身体疾病是为了与精神疾病对比。因此，正确答案为B。