What force do mountain climbers have to fight

Mountain warfare refers to warfare in the mountains or similarly rough terrain. This type of warfare is also called Alpine warfare, after the Alps mountains. Mountain warfare is one of the most dangerous types of combat as it involves surviving not only combat with the enemy but also the extreme weather and dangerous terrain.

Gravity because gravity pulls them to the ground

Lexy is correct because angle r is between the incident wave,  n  = 0, and a diffracted wave.