The wasabi served in most restaurants is dyed horseradish.
Why it is true
Real wasabi is rare and expensive. Thus, the spicy green paste that usually accompanies sushi is made from horseradish powder mixed with green food colouring.
Source: BBC
Would you rather be asked first?
This is the reading side of the site, so the answer is in the sentence above — that is the whole point of it. Test yourself works the other way round: the question comes on its own, and the fact and the explanation only appear once you have answered.