Question: The 2003 Auto Club 500 was won by the 2004 champion of what?
Related documents: [0]  [DOC] The 2003 Auto Club 500 was a NASCAR Winston Cup Series stock car race held on April 27, 2003 at California Speedway in Fontana, California. Contested over 250 laps on the 2-mile (3.23 km) asphalt D-shaped oval, it was the tenth race of the 2003 NASCAR Winston Cup Series season. Steve Park of DEI won the pole. Kurt Busch of Roush Racing won the race. This was also the last race for Jerry Nadeau. His career ended with an injury during practice at Richmond [END] 
[1]  [DOC] Kurt Thomas Busch (born August 4, 1978) is an American professional stock car racing driver. He currently drives the No. 41 Ford Fusion for Stewart-Haas Racing in the Monster Energy NASCAR Cup Series. He was the 2004 NASCAR Nextel Cup Series champion. Busch is currently a free agent for 2018, but Stewart-Haas Racing expects him back. He is the older brother of 2015 NASCAR Sprint Cup Series champion Kyle Busch. The brothers are second-generation racing drivers; their father, Tom, won several NASCAR-sanctioned events. [END] 
Answer: 
NASCAR Nextel Cup Series
NASCAR Nextel Cup Series