A survey was conducted by a research team to investigate how the education level and years of experience are related to the annual income. Based on the survey's definition, education level is a categorical variable with five possible levels of "high school", "college", "some college", "university", and "graduate". How many dummy (indicators) variables the analyst should add to the regression model in order to capture "education level" as an independent variable?

a.

2

b.

5

c.

3

d.

4