What of the following factors was the most important reason why Texas ranchers were motivated to drive their cattle north to Kansas?

Texas ranchers drove cattle north because demand and prices were higher in markets connected to eastern cities, so they could earn a large profit. The cattle drive itself was costly and difficult, but the higher selling price made the journey worthwhile. Getting rid of Texas fever was not the main motive; the disease actually made some northern states reluctant to accept Texas cattle.

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