You are writing a module that creates 6 EC2 instances using count, and you want to distribute them across a list of 3 availability-zone subnet IDs stored in var.subnet_ids. You need each instance's subnet_id to cycle through the 3 subnets (instance 0 → subnet 0, instance 3 → subnet 0 again, etc.) without going out of bounds. Which expression should you assign to subnet_id?