Please choose the correct answer to the question: The water contained in a geyser system gains energy from the underground material surrounding it. The water molecules gain kinetic energy and this results in an increase in the pressure of the water. Eventually the geyser erupts and expels water into the air above ground. Which of the following types of energy is the source for the initial energy gain of the water?

Answer1: electrical Answer2: magnetic Answer3: mechanical Answer4: thermal

Answer format: answer1/answer2/answer3/answer4
Answer: answer4