Alfred Crosby said he came upon the expression "Columbian Exchange" because

The impact of living organisms traded between the Old and the New World.

Explanation:

Historian Alfred W. Crosby came with the term “Columbian Exchange” to describe the impacts of the living organisms begin in the 15th century. With the discovery of America, in 1492, led the beginning of the Exchange between two continents. There was an exchange of native animals, plants, and diseases moving back and forth across the oceans. Europeans made trading connections with the natives and established colonies. The invaders came in with infectious diseases in the New World, that led to the decline of the entire civilizations. The Native American died with coming connecting with chickenpox, smallpox, scarlet fever, cholera, malaria, measles, etc.

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