What are the possible causes of this problem?

Bob the WLAN administrator is troubleshooting an IPsec VPN problem that has been deployed as the security solution over a point-to-point 802.11 wireless bridge link between two buildings. Bob cannot get the VPN tunnel to establish and notices that there is a certificate error during the IKE Phase 1 exchange.

What are the possible causes of this problem? (Choose all that apply.)
A . The VPN server behind the root bridge is using AES-256 encryption, and the VPN endpoint device behind the nonroot bridge is using AES-192 encryption.
B . The VPN server behind the root bridge is using SHA-1 hash for data integrity, and the VPN endpoint device behind the nonroot bridge is using MD-5 for data integrity.
C . The root CA certificate installed on the VPN device behind the nonroot bridge was not used to sign the server certificate on the VPN server behind the root bridge.
D . The clock settings of the VPN server that is deployed behind the root bridge predate the creation of the server certificate.
E . The public/private IP address settings are misconfigured on the VPN device behind the nonroot bridge.

Answer: CD