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As of the early 21st century , it was estimated that four billion people live primarily on a plant-based diet , some because of limits caused by shortages of cropland , freshwater , and energy resources .
 time: 21st century
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: 21st century 
Answer:
 time: 21st century