Please choose the correct answer to the question: Rolando wants to see at what temperature various liquids boil. He boils tap water and records the temperature. He then boils tap water with milk added and tap water with sugar added. What is the purpose of boiling plain tap water first?

Answer1: to include a factor to manipulate Answer2: to have a possible explanation to a problem Answer3: to change one variable while observing the others Answer4: to provide a standard by which to compare the results

Answer format: answer1/answer2/answer3/answer4