When the starter’s gun went off for the 200-meter final in the Olympic Games on Thursday, American sprinter Marion Jones took a substantial lead so quickly that it looked as though her competitors weren’t in the race at all. The result: Jones won by the biggest margin in that event in 40 years, sailing across the finish line in 21.84 seconds, which was 0.43 seconds faster than her closest competitor, Pauline Davis-Thompson of the Bahamas. Jones’s victory gave her her second gold medal, but her so-called “drive for five” gold medals was ended when she won the bronze in Friday’s long jump competition. Jones, who will compete for more gold in the 4×100 and 4×400 relays this weekend, said she tried to be unaffected by the controversy surrounding her shot-putter husband, C.J. Hunter, who is fighting charges that he tested positive for steroids. “It was something that could easily have swayed my focus and effort, but didn’t,” she told reporters after her win.