Click on the graph to show the correct relationship between altitude and boiling point
Altitude
Boiling Point Temp.

Explanation:

This graph shows that an increase in altitude causes a relative decrease in  boiling point temperatures. The two variables, it can be said, are inversely proportional.

This is because, with an increase in altitude, there is a proportionate decrease in air pressure. This means the vapor pressure of the fluid becomes strong enough, at a relatively lower temperature, to overcome the air pressure and for the liquid to boil.

This is why water can even boil without inputting heat into it but rather by just reducing the ambient air pressure.