Please choose the correct answer to the question: In which example could a thermometer be used to show that heat energy is being transferred?

Answer1: A piano is played by striking the keys. Answer2: A shovel is used to move dirt from one place to another. Answer3: A ball is thrown into the air. Answer4: A stove is used to boil water in a pan.

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