 Complete the following phrase:  1968 turned out to be a turning point in the history of Formula One, in terms of technicalities and safety. Wings were used on Formula One cars and aerodynamics really played a part in terms of the cars performance, and 5 Grand Prix drivers were killed in this year  including Jim Clark, Mike Spence, Jo Schlesser and Ludovico Scarfiotti  Clark at a Formula 2 race at Hockenheim in a Lotus in April, Spence during practice for the Indianapolis 500 in a Lotus in May, Scarfiotti during a hillclimb event in Germany driving a Porsche sportscar in June, and Schlesser during the French Grand Prix driving a Honda in July. It was the last year where all the races were run on tracks with almost no safety modifications. The rather dubious events of the season included Schlessers almost recklessly caused fatal accident at 