{{i}}quEstions 18～21 ArE BAsED on thE Following DiAloguE.{{/i}}

whAt is thE mAin rEAson thAt thE mAn runs EvEry DAyA．to think out somE DiFFiCult proBlEms.
B．to Do somE Cross-Country running.
C．to Finish A CoursE in physiCAl trAining.
D．to kEEp Fit AnD hEAlthy.