1. 8531. A certificated mechanic shall not exercise the privi leges of the certificate and rating unless, within the pre ceding 24 months, the Administrator has found that the certificate holder is able to do the work or the certificate holder has A. served as a mechanic under the certificate and rating for at least 18 months B. served as a mechanic under the certificate and rating for at least 12 months. C. served as a mechanic under the certificate and rating for at least 6 months.