Which of the following is not a violation of the SEC's (PCAOB's) auditor independent rules? a. The partner-in-charge of an audit rotates off the audit to another audit partner every seven years. b. The audit client's CEO was an audit partner of the external auditor firm six months ago. c. The external auditor firm rotates the audit to another external auditor firm every seven years d. The audit client's CFO came from the external auditor firm six months ago