a stopwatch. a heat source. safety goggles. a thermometer.
I gave my students this multiple choice question: To determine the boiling point of a liquid, a student must use all of these tools except

Only one answer is correct among these 4 choices:
- a stopwatch.
- a heat source.
- safety goggles.
- a thermometer.

Could you tell me which one is correct?

a stopwatch.