Extract the years that Ayrton Senna da Silva won the Formula One World Championship, and put them separated by comma.

Ayrton Senna da Silva (21 March 1960 – 1 May 1994) was a Brazilian racing driver who won the Formula One World Drivers' Championship in 1988, 1990, and 1991. Senna is one of three Formula One drivers from Brazil to win the World Championship and won 41 Grands Prix and 65 pole positions, with the latter being the record until 2006.

1988, 1990, 1991