Evaporation is a phenomenon in which water turns into gaseous form without reaching its boiling point. It rather depends on other factors such as wind speed, heat, surface etc.The water could not have dissapeared due to heat as it is illogical to think of the temperature reaching its boiling point.
All the other options are illogical and hence evaporation is the correct answer.
Because when you add 27 and 39 together you get 66. And the interior of a triangle equals 180 so now you must subtract the 66 from 180. And once you do that you get 114 degrees.