measuring the amount of fatigue making sure the same foods are eaten recording observations in the same chart making sure the foods are at the same temperature
I gave my students this multiple choice question: Janette conducts an investigation to see which foods make her feel more fatigued. She eats one of four different foods each day at the same time for four days and then records how she feels. She asks her friend Carmen to do the same investigation to see if she gets similar results. Which would make the investigation most difficult to replicate?

Only one answer is correct among these 4 choices:
- measuring the amount of fatigue
- making sure the same foods are eaten
- recording observations in the same chart
- making sure the foods are at the same temperature

Could you tell me which one is correct?

measuring the amount of fatigue