That or Which

http://www.chass.utoronto.ca/~cpercy/courses/6362-lee.htm
・prescriptive には、that は限定用法のみ、which は非限定用法のみ。
・現在の日常の用法では、カンマやポーズがあれば非限定、prescriptive の傾向も残る

(3) I want her to give me back the book that I loaned her a year ago.
(4) Kumar will never return his gold medal, which he believes he won fairly.