In general, who refers to people, and that andwhich refer to animals and things. That mayalso be used to refer to an anonymous group of people ("I want tocontribute to a group that saves dolphins").
Professor Thomas, who has taught at theuniversity for forty years, is also a ranger at the local statepark.
The whale that swam into the bay wassuccessfully returned to the ocean.
The plane, which flew at an altitude of 30,000feet, arrived in only 7 hours.
That is used only with restrictiveclauses, and which can be used with either restrictiveor nonrestrictive clauses, although somewriters prefer to use it only with nonrestrictive elements.
A factory that does not take care of itswaste stream may violate environmental laws.[restrictive]
After Suzanne left the factory, which isone of the most polluted Superfund sites in the country, she ranfor Congress. [nonrestrictive]
See also who/whom.
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