Which statement about SNMP is true？（）
- ASNMP version 2 uses a proxy agent to forward GetNext message to SNMP version 3.
- BSNMP version 2 supports message integrity to ensure that a packet has not been tampered with
- CProxy agents were used only in SNMP version 1.
- DSNMP version 3 supports encryption and SNMP version 2 support authentication.
- EGetBulk messages are converted to GetNext messages by the proxy agent and are then forwarded to the SNMP version 1 agent.