Question

People who live at sea level often have trouble breathing at the tops of high mountains

Answer 1

The altitude messes with people and they aren't used to breathing the air

Answer 2

People who live at sea level often have trouble breathing at the tops of high mountains because of ALTITUDE SICKNESS. Air at high altitude have less oxygen than air at sea level. Due to lack of oxygen, people feel dizzy and have trouble in breathing.

Hope it helps you:)