Question: Sacramento Mather Airport is located on the site of a former air force base, located 12 mi east of Sacramento, on the south side of U.S. Route 50 in Sacramento County, California, which was closed in what year?
Sentence (from "Mather Air Force Base"): Mather Field was one of 32 Air Service training camps established after the United States entry into World War I in April 1917.
Relevant evidence: No