You are given a paragraph, with numbered sentences, and a question. Write the numbers of the sentences needed to answer the given question. Separate multiple sentence numbers with a comma (", "). E.g. "1, 2" would be the correct response if Sent 1 and Sent 2 are needed to answer the given question (do not include sentences that contain no information that is necessary to answer the question). Sort the sentence numbers in an increasing order (i.e., '1, 2', instead of '2, 1'). Your answer should not contain anything than the sentence numbers separated by a comma (e.g. '1, 2, 3, 4'). The given question requires more than one sentence to answer. So, your answer must contain at least one comma (", ").

Example input: Paragraph- Sent 1: A Republican bid to keep the government open past Friday includes no new money for the construction of a border wall along the U.S.-Mexico border, according to several congressional aides familiar with ongoing talks.
Sent 2: GOP leaders submitted the new offer Tuesday afternoon in an effort to appease Democrats, whose votes are needed to avert a shutdown of federal agencies, several House and Senate aides said.
Sent 3: However, Republicans also insisted on increases in border security and defense spending, including money to repair existing fencing and new surveillance technology to patrol the nearly 2,000-mile border.
Sent 4: Democrats have indicated that they would support such appropriations so long as no money goes toward an actual wall.
Question: Who has to be appeased to keep the government open?.
Example output: 1, 2.
Example explanation: The question is: Who has to be appeased to keep the government open? The phrase "appease Democrats" is mentioned in Sent 2, about accepting offer that is mentioned in Sent 1 to "keep the government open". So, Sent 1 and Sent 2 are the sentences needed to answer this question.
Q: Paragraph- Sent 1: On 9/11, the defense of U.S. airspace depended on close interaction between two federal agencies: the FAA and the North American Aerospace Defense Command (NORAD).
Sent 2: The most recent hijacking that involved U.S. air traffic controllers, FAA management, and military coordination had occurred in 1993.90 In order to understand how the two agencies interacted eight years later, we will review their missions, command and control structures, and working relationship on the morning of 9/11.
Sent 3: FAA Mission and Structure.
Sent 4: As of September 11, 2001, the FAA was mandated by law to regulate the safety and security of civil aviation.
Sent 5: From an air traffic controller's perspective, that meant maintaining a safe distance between airborne aircraft.
Sent 6: Many controllers work at the FAA's 22 Air Route Traffic Control Centers.
Sent 7: They are grouped under regional offices and coordinate closely with the national Air Traffic Control System Command Center, located in Herndon, Virginia, which oversees daily traffic flow within the entire airspace system.
Sent 8: FAA headquarters is ultimately responsible for the management of the National Airspace System.
Sent 9: The Operations Center located at FAA headquarters receives notifications of incidents, including accidents and hijackings.
Sent 10: FAA Control Centers often receive information and make operational decisions independently of one another.
Sent 11: On 9/11, the four hijacked aircraft were monitored mainly by the centers in Boston, New York, Cleveland, and Indianapolis.
Sent 12: Each center thus had part of the knowledge of what was going on across the system.
Sent 13: What Boston knew was not necessarily known by centers in New York, Cleveland, or Indianapolis, or for that matter by the Command Center in Herndon or by FAA headquarters in Washington.
Sent 14: Controllers track airliners such as the four aircraft hijacked on 9/11 primarily by watching the data from a signal emitted by each aircraft's transponder equipment.
Sent 15: Those four planes, like all aircraft traveling above 10,000 feet, were required to emit a unique transponder signal while in flight.
Sent 16: On 9/11, the terrorists turned off the transponders on three of the four hijacked aircraft.
Sent 17: With its transponder off, it is possible, though more difficult, to track an aircraft by its primary radar returns.
Sent 18: But unlike transponder data, primary radar returns do not show the aircraft's identity and altitude.
Question: Where is the FAA's Air Traffic Control System Command Center located ?.
A:
6, 7.