41911
In 1953 , REM sleep was discovered as distinct , and thus William C. Dement and Nathaniel Kleitman reclassified sleep into four NREM stages and REM .
 person: William C. Dement; person: Nathaniel Kleitman
Given options, please tell me the categories of all the listed entity words.Output format is "type1: word1; type2: word2". 
Option: media, plant, vehicle, astronomical object, time, disease, animal, location, person, organization, mythical figure, food, biology, geographical phenomenon, instrument, event 
Text: {0} 
 Entities: William C. Dement, Nathaniel Kleitman 
Answer:
 person: William C. Dement; person: Nathaniel Kleitman