WAEC - Physics (1989 - No. 14)
Water in an open container boils at a lower temperature when heated at the top of a mountain than at sea-level because at the top of a mountain the
relative humidity is higher than at sea-level
rays of the sun adds more heat to the water
temperature is lower than that at sea-level
pressure is lower than that at sea-level
pressure is more than that at sea-level
Uitleg
Refer to Elementary physics, pressure reduces with height.