The user found that there are many IPs starting with 100 frequently accessing the back-end ECS instance of their ELB, and the security check did not report the problem. Which of the following is the possible reason?

The user found that there are many IPs starting with 100 frequently accessing the back-end ECS instance of their ELB, and the security check did not report the problem. Which of the following is the possible reason?
A . ELB system health check and availability monitoring
B . Access by real users
C . Being under cyber attack.
D . ECS server failure

Answer: A