The 712th Expeditionary Air Refueling Squadron is an inactive United States Air Force unit.
 It was last assigned to the 19th Operations Group at Robins Air Force Base, Georgia, where it was inactivated on 1 July 1996.
 In February 2001, the squadron was converted to provisional status as the 712th Expeditionary Air Refueling Squadron.
Mather Air Force Base (Mather AFB) was a United States Air Force Base, which was closed in 1993.
 It was located 12 mi east of Sacramento, on the south side of U.S. Route 50 in Sacramento County, California.
 Mather Field was one of 32 Air Service training camps established after the United States entry into World War I in April 1917.
The 28th Expeditionary Air Refueling Squadron is a provisional United States Air Force unit.
 It was last known to be assigned to the 40th Air Expeditionary Group.
 Diego Garcia Air Base, British Indian Ocean Territory.
 Its current status is undetermined
The 904th Expeditionary Air Refueling Squadron is an inactive United States Air Force unit.
 It was last assigned to the 320th Bombardment Wing at Mather Air Force Base, California, where it was inactivated on 30 September 1986.
The 908th Expeditionary Air Refueling Squadron is a provisional United States Air Force (USAF) unit.
 It is assigned to the 380th Expeditionary Operations Group at Al Dhafra Air Base in United Arab Emirates.
 It has supported operations in Afghanistan, Iraq, and Syria from this location.
 The squadron has a varied background, having been formed by a series of consolidations of no fewer than five distinct units.
The 310th Expeditionary Air Refueling Flight is a provisional United States Air Force unit.
 The flight was last active as the 310th Air Refueling Squadron in October 1994, when it was assigned to the 380th Air Refueling Wing at Plattsburgh Air Force Base.
The 44th Expeditionary Air Refueling Squadron is a provisional United States Air Force unit attached to the 379th Air Expeditionary Wing.
 It was last known to be stationed at Al-Udeid Air Base, Qatar, where it engaged in air refueling operations in support of United States Air Forces Central.
The 340th Expeditionary Air Refueling Squadron is a provisional United States Air Force unit.
 It is assigned to the 379th Expeditionary Operations Group at Al Udeid Air Base, Qatar.
 It has supported operations in Afghanistan, Iraq, and Syria from this location.
The 90th Expeditionary Air Refueling Squadron is a provisional United States Air Force unit.
 Until March 2013, it was assigned to the 385th Air Expeditionary Group at Incirlik Air Base, Turkey.
 It then departed for an undisclosed location in Southwest Asia.
The 900th Expeditionary Air Refueling Squadron is a provisional United States Air Force unit.
 It was last known to be assigned to the 398th Air Expeditionary Group at Incirlik Air Base, Turkey.

Question: The 904th Expeditionary Air Refueling Squadron was last assigned at a base which closed in which year ?
Answer: 1993