Read this: Multiracial people who wanted to acknowledge their full heritage won a victory of sorts in 1997, when the Office of Management and Budget (OMB) changed the federal regulation of racial categories to permit multiple responses. This resulted in a change to the 2000 United States Census, which allowed participants to select more than one of the six available categories, which were, in brief: "White," "Black or African American," "Asian," "American Indian or Alaskan Native," "Native Hawaiian or other Pacific Islander," and "Other." Further details are given in the article: Race (U.S. census). The OMB made its directive mandatory for all government forms by 2003.

In what year were participants no longer allowed to choose more than one of the six available categories?
What is the answer? (If it cannot be answered, return "unanswerable")
unanswerable