Being an incumbent president or judge has the advantage that one is widely recognized: this means that the name, image and face will be recognized by the voters - the correct answer is b.
Being an incumbent president is not in particular related to any partt.
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.
