Simplify the following text for a Grade 8 native English-speaking person.
Original: The Marcus collections included significant works by Mexican artists Rufino Tamayo , David Alfaro Siqueiros , Diego Rivera , and Rivera 's lesser-known friend and colleague Antonio RuÃ z ; the American sculptor Alexander Calder , and American painter Georgia O'Keeffe .
Simplified: His favorite artists from Mexico were Rufino Tamayo , David Alfaro Siqueiros , Diego Rivera , and Antonio RuÃ z. He became friends with Rivera and Tamayo . Mr. Marcus also liked American artists , such as Alexander Calder , who made sculptures , and Georgia O'Keeffe , who was a painter .