Which of the following statements is correct? With respect to efficien

Which of the following statements is correct? With respect to efficiency wage models,

a. their key element is an explanation of why the efficiency (or productivity) of workers depends on the real wage.
b. the rationale underlying those models implies that firms will set the real wage above the market clearing level.
c. they explain a real wage rigidity.
d. all of the above

ANSWER

D