Answer:
They forgot to measure the amount of water poured into each container.
Step-by-step explanation:
When an experiment is set up, all variables must be established in the same proportion for all experimental units. In this case, for the experiment to be efficient and present true results, it is important that the paper towel strips have the same size, the containers where they will be inserted are the same and they are tested in the same amount of water, that is, each container must contain the same amount of water. However, the amount of water was not measured in the above experiment and this was the variable that the researchers forgot to control, which will harm the veracity of the results.