Complete the sentences below with 'a', 'an', or 'No article'
The Smithsonian hence became a group of museums and research centers administered by the government of the United States. It became known as "the nation's attic" for its eclectic holdings of 138 million items. The Institution's nineteen museums, nine research centers, and zoo include historical and architectural landmarks, mostly located in Washington D.C. The Institution's thirty million annual visitors are admitted without charge.