Which of the following statements is incorrect? Select one: 

a). When there is involuntary unemployment, the employment rent is positive. 

b). There must be involuntary unemployment when the firms pay workers a wage lower than the workers' reservation option. 

c). When a firm sacks a worker against her wishes and she cannot get another job immediately, there is involuntary unemployment. 

d). When there is involuntary unemployment, firms must be firing employees who do not work hard enough.