Teacher: In this task, you're given a context passage, followed by a question that needs to be answered. Based on the paragraph, you must write unambiguous answers to the questions and your answer must refer to a specific phrase from the paragraph. If multiple answers seem to exist, write the answer that is the most plausible.
Teacher: Now, understand the problem? If you are still confused, see the following example:
Tracy slept awkwardly on their new bed and was having some pain, so Tracy cracked her neck. 
Question: What did Tracy do with her neck?
Solution: crack her neck
Reason: The output phrase perfectly answers the question and is explicitly mentioned in the passage as well.

Now, solve this instance: In July 1969, astronaut Jim Lovell hosts a house party where guests watch Neil Armstrong's televised first human steps on the Moon. Afterwards Lovell, who had orbited the Moon on Apollo 8, tells his wife Marilyn that he intends to return to the Moon to walk on its surface. Three months later, as Lovell conducts a VIP tour of NASA's Vertical Assembly Building, his boss Deke Slayton informs him that because of problems with Alan Shepard's crew, his crew will fly Apollo 13 instead of 14. Lovell, Ken Mattingly, and Fred Haise train for their new mission. A few days before launch, Mattingly is exposed to the measles, and the flight surgeon demands his replacement with Mattingly's backup, Jack Swigert. Lovell resists breaking up his team, but relents when Slayton threatens to bump his crew to a later mission. As the launch date approaches, Marilyn has a nightmare about her husband getting killed in space, but goes to the Kennedy Space Center the night before launch to see him off. On April 11, 1970, Flight Director Gene Kranz gives the go-ahead from Houston's Mission Control Center for the Apollo 13 launch. As the Saturn V rocket climbs through the atmosphere, a second stage engine cuts off prematurely, but the craft reaches its Earth parking orbit. After the third stage fires to send Apollo 13 to the Moon, Swigert performs the maneuver to connect the Command/Service Module Odyssey to the Lunar Module Aquarius and pull it away from the spent rocket. 
Question: Which astronaut was replaced by Swigert?
Student:
Mattingly