Q:Generate a question that has the following answer: twenty for the following movie plot: Ash Ketchum, Brock and Dawn are at a Pokemon Center watching a Pokemon Baccer match on TV. Ash and crew are intrigued by the match and decide to head to Crown City to watch the Pokemon Baccer World Cup. Meanwhile, businessman Grings Kodai, his henchman Goone and assistant Rowena are in Unova where they find Zoroark and a sleeping Zorua. They capture the two Pokemon and take off for Crown City. Zorua, who is trapped on Kodai's plane, manages to escape and disguises itself as a Skiploom, and drifts down to the ground unnoticed. Meanwhile, the protagonists are on their way to the Pokemon Baccer World Cup in Crown City, and encounter and rescue the Zorua from a group of Vigoroth. Zorua is able to speak to humans through telepathy. Zorua wishes to go to Crown City to find its mother Zoroark who it refers to as "Meema". Zorua's shapeshifting skills are compromised by the constant presence of its tail; touching it reverts Zorua to normal. The next day, Celebi appears in town and causes all the flowers to bloom, much to the delight of several townsfolk, who recall a mysterious event that happened twenty years where all the city's flora died. Meanwhile, Kodai unleashes the imprisoned Zoroark upon the city, pretending to hold Zorua prisoner. Zoroark uses her illusion powers to take the form of Raikou, Entei and Suicune and scare the townsfolk; Kodai edits recorded footage of her to make it look like the fake Pokemon are on the rampage and labels Zoroark as evil. Ash, Dawn, Brock and Zorua arrive, seeing Zoroark on television, Zorua trying to reach the cut-off town. They meet Karl, a local journalist investigating Kodai, who leads them into town through a hidden passage in a windmill. Kodai is revealed to be able to see into the future, caused by an encounter with Celebi twenty years ago where he touched the Time Ripple, created when Celebi time travels, gaining his powers of foresight, causing the destruction of the town's plant life in the process. Kodai is looking for the Time Ripple again, and has Goone...
A:
How many years was Celebi gone for?