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.
I think its because most of the people that say that have never been to Africa. And they dont know what they are talking about. If you went to Africa you would understand.