A SysOps Administrator created an Application Load balancer (ALB) and placed two Amazon EC2 instances in the same subnet behind the ALB. During monitoring, the Administrator observes HealthyHostCountdrop to 1 in Amazon CloudWatch.
What is MOST likely causing this issue?

A. The EC2 instances are in the same Availability Zone, causing contention between the two.
B. The route tables are not updated to allow traffic to flow between the ALB and the EC2 instances.
C. The ALB health check has failed, and the ALB has taken EC2 instances out of service.
D. The Amazon Route 53 health check has failed, and the ALB has taken EC2 instances out of service.