Answer:
"Referees must work years before refereeing the World Cup."
Step-by-step explanation:
"Referees are tired after a game." Maybe, but not the point.
"Each official has to be able to run backwards." That's for them 'perfect their fitness and endurance'
"They are refereeing games with expert players." The paragraph is focusing on referees, not if they're playing with expert playes, but this could also be the answer but I'm not sure how.
Good luck, hope this helped! :)