You need to know the subtle difference between who and that. The best explanation I've read is in The New York Public Library Writer's Guide to Style and Usage. I think I paid A$56 (US$28) when on special at amazon.com This fantastic 800-page guide says:
Who and that are both used when referring to persons. The pronoun who is used to refer to an individual, and that is used to refer to a group of people:
- The exact number of American soldiers that participated in the Vietnam War has never been calculated.
- A politician is a man who understands government, and it takes a politician to run a government. A statesman is a politician who's been dead 10 or 15 years. (Quote from President Harry S Truman)