A jet stream forms high in the upper troposphere between two air masses<span> of very different temperature. The greater the temperature difference between the air masses, the faster the wind blows in the jet stream.
</span>.
.
So "Differences in temperature between two air masses" is the answer
(C.) The layers of the Earth are uniform in thickness
This is untrue. The mantle is far large than the crust and there are other large size differences.
The answer is (A) evaporation, because when water gets hot it turns into a gas and rises into the air.
The immune system, and the excretory system
(Honestly, all of them work to keep us 'healthy', but I believe these are the ones you're looking for)
