Article 40 of the Procedures for Handling Criminal Cases by Public Security Organs stipulates that public security organs should ensure that defense lawyers engage in the following professional activities in accordance with the law during the investigation stage: (1) To learn from the public security organs about the crimes and cases alleged by criminal suspects and give their opinions; (two) to meet and correspond with the criminal suspect, and to understand the case from the criminal suspect; (3) Providing legal aid and acting as an agent for complaints and accusations against criminal suspects; (4) applying for changing the compulsory measures against criminal suspects. Therefore, the answer to this question is ABCE.