A. The VPC peering connection has not been configured properly for the EC2 instance to communicate with the DB instance.
B. The route table of the private subnet that hosts the DB instance does not have a NAT gateway configured for communication with the EC2 instance.
C. The public subnet that hosts the EC2 instance does not have an internet gateway configured for communication with the DB instance.
D. The wrong security group was associated with the new DB instance when it was restored from the snapshot.

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