Detailed Instructions: In this task, you are given a question and a context passage. You have to answer the question based on the given passage.
Q: Which of the following is not a person's last name: Phillips, Mueller, or letter?, Context: The problems with North American were severe enough in late 1965 to cause Manned Space Flight Administrator George Mueller to appoint program director Samuel Phillips to head a "tiger team" to investigate North American's problems and identify corrections. Phillips documented his findings in a December 19 letter to NAA president Lee Atwood, with a strongly worded letter by Mueller, and also gave a presentation of the results to Mueller and Deputy Administrator Robert Seamans. Meanwhile, Grumman was also encountering problems with the Lunar Module, eliminating hopes it would be ready for manned flight in 1967, not long after the first manned CSM flights.
A:
letter