[found on wsuonline.weber.edu]

Who, Which, That:

“Do not use which to refer to persons. Use who instead. That, though generally used to refer to things, may be used to refer to a group or class of people.

• I just saw a boy who was wearing a yellow banana costume.
• I have to go to math next, which is my hardest class.
• Where is the book that I was reading?”

[found on http://wsuonline.weber.edu/wrh/words.htm]