Question:
Which employer did John W. Oswald work for in Apr, 1957?

Context:
 John W. Oswald works for University of Kentucky from Jan, 1963 to Jan, 1970. 
 John W. Oswald works for Pennsylvania State University from Jan, 1970 to Jan, 1983. 
 John W. Oswald works for University of California, Davis from Jan, 1946 to Jan, 1962.

Answer:
University of California, Davis