Answer:
C. Age is a probably lurking variable in this study.
Step-by-step explanation:
The two variables that are related in this study are arithmetic and exercise.
The variable which is not known and cannot be controlled is age. But it may effect other variables. Such a variable is called lurking variable.
In this experiment we know the age range but not exact age. It may effect as younger students may not be able to solve questions easily.
Choice C is the most appropriate conclusion given for the finding.