I'd say D. expert power because expert power is when an employee ( or in your case the teacher) has a high set of knowledge/skills that other employees do not posses. Which by the information above about this teacher's PhD and thier experience, I assume that this teacher must have expert power.
Underground water like in a well can't clean itself because of man made pollutants, from large farms that seep through the ground. there is little agitation to keep it areated, and becomes stagnant, which develops bacteria and other pathogens to make us sick..
