Imagine you work at Trinity Health Ann Arbor (formerly Trinity St. Joseph Mercy Hospital Ann Arbor) and are participating in a strategic planning process. You know that Washtenaw County is facing a significant nursing shortage. Despite several highly regarded nursing programs in the area, it is difficult to hire enough RN/BSN nurses to fill all of your vacancies. Evaluating the negotiating power that nurses have to command higher salaries and benefits in the current environment best corresponds to which of Porter's Five Forces?

a. New entrants to the field
b. Rivalry among existing firms
c. Substitute products or services
d. Customer bargaining power
e. Supplier bargaining power