At high altitudes there is greater atmospheric pressure than on the surface of the Earth. At high altitudes there is lower atmospheric pressure than on the surface of the Earth. If the cabin is not pressurized, ozone and other upper atmospheric gases will enter the airplane. If the cabin is not pressurized, carbon dioxide will escape from the airplane.
I gave my students this multiple choice question: Which of the following best explains why the pressure inside a high-flying airplane must be controlled?

Only one answer is correct among these 4 choices:
- At high altitudes there is greater atmospheric pressure than on the surface of the Earth.
- At high altitudes there is lower atmospheric pressure than on the surface of the Earth.
- If the cabin is not pressurized, ozone and other upper atmospheric gases will enter the airplane.
- If the cabin is not pressurized, carbon dioxide will escape from the airplane.

Could you tell me which one is correct?

At high altitudes there is lower atmospheric pressure than on the surface of the Earth.