Which statements relating to internal control is most correct? (Two options are correct in this question) I need more explanation about why options are correct or wrong.

A.The client's internal audit function is ignored for the purpose of understanding the client's internal control.

B.The control environment has a pervasive effect on assessing the likely effectiveness of internal controls.

C.Auditor's understanding of their client's internal controls is undertaken in order to assess the adequacy of internal controls for management purposes.

D.Auditor's understanding of their client's internal controls is necessary, irrespective of whether or not they intend to rely on their client's controls.