What causes the jet stream? A. Differences in altitude between two air masses B. Differences in temperature between two air masses C. Differences in size between two air masses D. Differences in shape between two air masses

A jet stream forms high in the upper troposphere between two air masses of very different temperature. The greater the temperature difference between the air masses, the faster the wind blows in the jet stream.

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So "Differences in temperature between two air masses" is the answer