FILE - In this April 20, 1972 black-and-white file photo, first lady Patricia Nixon welcomes China's giant pandas at Washington's National Zoo in Washington. With her is Ting-Hung, head of the Bureau of Public Service of the City of Peking, who accompanied the animals to Washington. The National Zoo is celebrating 40 years of pandas. Monday was the 40th anniversary of the day pandas Hsing-Hsing and Ling-Ling landed at Andrews Air Force Base in Maryland. The pandas were gifts to the United States from China following President Richard Nixon's historic visit to the country. The pandas were officially presented to the Zoo on April 20, 1972. Ling-Ling lived at the National Zoo until 1992, when she died. Hsing-Hsing died in 1999. (AP Photo, File)