A SysOps Administrator is maintaining an application that runs on Amazon EC2 instances behind an Application Load Balancer (ALB). Users are reporting errors when attempting to launch the application. The Administrator notices an increase in the HTTPCode_ELB_5xx_CountAmazon CloudWatch metric for the load balancer.
What is a possible cause for this increase?

A. The ALB is associated with private subnets within the VPC.
B. The ALB received a request from a client, but the client closed the connection.
C. The ALB security group is not configured to allow inbound traffic from the users.
D. The ALB target group does not contain healthy EC2 instances.