Here's a problem to solve: Students want to test how the temperature changes each day throughout the year. Which of the following does not have to be kept the same during the test?

Among the 4 following options, which is the correct answer?

- A: the place where the temperature is measured
 
- B: the thermometer used to take the measurement
 
- C: the person reading the temperature measurement
 
- D: the time of day when the temperature is measured
 