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.
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)