Final answer:
Wayne Gretzky joining the NHL
Step-by-step explanation:
The question is referring to a specific event in hockey history, so the subject is Sports. The grade level is High School.
In the 1979-80 season, Wayne Gretzky would join the National Hockey League (NHL) and play for the Edmonton Oilers. He had an exceptional career and went on to become one of the greatest hockey players of all time.
Therefore, the blank in the sentence should be filled with 'joining the NHL' or 'playing for the Edmonton Oilers' or 'starting his professional career'.