CARSON CITY (AP) -- Invoking national security, President Bush has renewed an exemption allowing the Air Force to keep mum about its top-secret operations at Groom Lake, also known as Area 51, in southern Nevada. Bush signed a memorandum declaring it of "paramount interest'' to exempt the base from disclosing classified information. President Clinton first issued the exemption in 1995 in response to two lawsuits filed by injured workers in Nevada seeking information about the military's environmental practices at the site. It has been renewed yearly. The government has acknowledged the existence of the Groom Lake installation but has not...