I must be blanking ... "CC"?
To be totally accurate, the auditor is hired by the Board of Directors of the corporation. A bid process usually only happens when there is a change of auditor and it is typically awarded to the lowest bidder. However, they are not done on an annual basis. Many corporations retain the same auditor for several years in a row. Don't know what the facts are with Enron. My guess is that AA has been their auditor for some time.