Explain simply why 1931 is the correct answer to: Passage: Urbanisation began in 1902, when a railroad linking Los Angeles and Salt Lake City attracted many farmers to the area, and fresh water was piped into the settlement. In 1911, the city was incorporated as part of Clark County. In 1931 work started on the Boulder Dam (now the Hoover Dam), bringing a huge influx of young male workers, for whom theatres and casinos were built, largely by the Mafia. Electricity from the dam also enabled the building of many new hotels along the Strip. The arrival of Howard Hughes in 1966 did much to offset mob influence, and helped turn Las Vegas into more of a family tourist centre, now classified as a Megaresort.

Question: Based on this passage, when did las vegas become a gambling city?. Explanation:
This is the relevant information: In 1931 work started on the Boulder Dam (now the Hoover Dam), bringing a huge influx of young male workers, for whom theatres and casinos were built, largely by the Mafia.