to include a factor to manipulate to have a possible explanation to a problem to change one variable while observing the others to provide a standard by which to compare the results
I gave my students this multiple choice 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?

Only one answer is correct among these 4 choices:
- to include a factor to manipulate
- to have a possible explanation to a problem
- to change one variable while observing the others
- to provide a standard by which to compare the results

Could you tell me which one is correct?

to provide a standard by which to compare the results