All of the following are employer practices unfair to labor under the National Labor Relations Act EXCEPT: a. Interference with the efforts of employees to form, join, or assist labor organizations or to engage in concerted activities for their mutual aid or protection. b. An employer's domination of a labor organization or contribution of financial or other support to it. c. Discrimination in the hiring of or the awarding of tenure to employees for reason of union affiliation. d. Discrimination in the payment of wages and benefits according to the skill and ability of the workers employed by the employer. e. Discrimination against employees for filing charges under the act or giving testimony under the act. f. Refusal to bargain collectively with the duly designated representative of the employees.