(a) Each promotional examination is open to each police officer who for at least two years immediately before the examination date has continuously held a position in the classification that is immediately below, in salary, the classification for which the examination is to be held.
(b) If the department has adopted a classification plan that classifies positions on the basis of similarity in duties and responsibilities, each promotional examination is open to each police officer who has continuously held for at least two years immediately before the examination date a position at the next lower pay grade, if it exists, in the classification for which the examination is to be held.
(c) If there are not sufficient police officers in the next lower position with two years' service in that position to provide an adequate number of persons to take the examination, the commission shall open the examination to persons in that position with less than two years' service. If there is still an insufficient number, the commission may open the examination to persons in the second lower position, in salary, to the position for which the examination is to be held.