Demographers Predict That the Average Age of the Population of the U.S.will

Demographers predict that the average age of the population of the U.S.will increase.One result will be an increase in the demand for nurses.Assume that the graph below describes the current nursing labor market in a mid-sized city,and that the market is a competitive one.
Suppose that several years ago the supply of nurses in this city was as shown as Supply A.What might explain the change from Supply A to Supply,current?

A) an increase in nurses' wages.
B) a decrease in nurses' wages.
C) increased immigration of people trained as nurses in other countries.
D) increased employment opportunities for people who would have once chosen nursing as a career.