(a) The contributions required by this part shall not apply to any amount of the aggregate compensation paid to such employee by all such employers in such calendar month which is in excess of the MCB; and

(b) Each employer (other than a subordinate unit of a national-railway-labor-organization employer) shall be liable for that portion of the contribution with respect to such compensation paid by all such employers which the compensation paid by the employer to such employee bears to the total compensation paid in such month by all such employers to such employee.

(c) In the event that the compensation paid by such employers to the employee in such month is less than the MCB, each subordinate unit of a national-railway-labor-organization employer shall be liable for such portion of any additional contribution as the compensation paid by such employer to such employee in such month bears to the total compensation paid by all national-railway-labor-organization employers to such employee in such month.