Simplify the following text for a Grade 11 native English-speaking person.
Original: Clarissa Harlowe `` Clara '' Barton ( December 25 , 1821 -- April 12 , 1912 ) was a pioneer American teacher , nurse , and humanitarian .
Simplified: Clarissa Harlowe `` Clara '' Barton ( December 25 , 1821 â `` April 12 , 1912 ) was an American teacher , nurse , and humanitarian .