AU$-$C $330$ Performing Audit Procedures in Response to Assessed Risks and Evaluating the Audit Evidence Obtained and AS $2301$ The Auditor's Responses to the Risks of Material Misstatement state that auditors a$)$are required to perform substantive procedures for all relevant assertions that have been identified after issuance of the audit opinion b$)$are required to perform substantive procedures for all relevant assertions that have been identified during the risk assessment phase c$)$are required to perform tests of controls for all relevant assertions that have been identified during the risk assessment phase d$)$should notify the client immediately of a change in fee structure pertaining to the audit