The regional variation in the appearance of humans in clustered populations probably results a from a long history of adaptation to different environments. b development of populations on different continents. c territoriality kept people from mixing and intermarrying. d wars kept populations separated.

a. long history of adaptation to different environments.

Explanation:

The regional variation in the appearance of humans in clustered populations probably results from a " long history of adaptation to different environments."

The above statement is true in the sense that a "long history of adaptation to different environments" can lead to some of the following in organisms, and specifically humans in this case.

1. It fits humans to their immediate and physical environment such that there is improvement in the human evolutionary condition.

2. It creates adaptive traits that is strengthened as a result of natural selection.

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