A map of North America with international borders, state borders and province borders is shown. The map legend displays four types of borders. State and province borders are shown as a grey line and international borders are shown with a thick dark line. State river boundaries are shown with a thin blue line and international river boundaries are shown with a thick blue line.
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Which of the following borders is not defined by a natural boundary?
The state border between Arkansas and Mississippi
The state border between Texas and Oklahoma
The national border between Texas and Mexico
The national border between Montana and Canada