Which of the following statements is correct?

Select one:

a.When there is involuntary unemployment, the employment rent is zero.

b.When a firm sacks a worker against her wishes but she can get another job immediately, there is involuntary unemployment.

c.There must be involuntary unemployment when the firms pay workers a wage higher than the workers' reservation option.

d.When there is no involuntary unemployment, firms must be employing workers without ever dismissing them.