The answer in the space provided is experiments. It is because in order to produce causation or to likely have a cause of something in particular, they need to conduct experiments in which they will likely form new ideas and test this ideas to form something new that will cause and contribute to others and will be of gain of knowledge.
The case of the extraneous variable is most likely to be determined by the student grade level that is level of the education program that is for the grade level. This refers to all the course level and can be use to trace out the external variable that are not intentionally studied in the test or experiment and tends to affect the other variables.
