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One dominant theory holds that the canyon is a remnant of an ancient outlet of the Colorado River which once existed before the Gulf of California opened up about 7.9 million years ago .
 Colorado River, Gulf of California
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: media, plant, vehicle, astronomical object, time, disease, animal, location, person, organization, mythical figure, food, biology, geographical phenomenon, instrument, event 
Text: {0} 
Answer:
 Colorado River, Gulf of California