You have set up an Elastic Load Balancer (ELB) to distribute traffic among several EC2 instances. However, the ELB has flagged all of your EC2 instances in the target group as unhealthy. Despite this, you are still able to access the website directly by entering the IP addresses of the EC2 instances into your web browser.

What might be the causes for the ELB marking your instances as unhealthy? (Select two)