Ansible synchronize module delegate_to could not resolve hostname - ansible

Question

I am using synchronize module to transfer file form serverA to server B. My serverA and serverB hosts are:
[serverB]
172.20.13.201 ansible_user=root ansible_ssh_pass="hi12#$"
172.20.13.202 ansible_user=root ansible_ssh_pass="hi12#$"
172.20.13.203 ansible_user=root ansible_ssh_pass="hi12#$"
[serverA]
172.20.5.121 ansible_user=root ansible_ssh_pass="hi12#$"
my ansible playbook is
- hosts: serverB
tasks:
- name: Copy Remote-To-Remote
remote_user: root
synchronize: src=/root/connection dest=/root/neutron-server.log
delegate_to: serverA
But it error
TASK [Copy Remote-To-Remote] ***************************************************
fatal: [172.20.13.201]: UNREACHABLE! => {"changed": false, "msg": "Failed to connect to the host via ssh: ssh: Could not resolve hostname servera: nodename nor servname provided, or not known", "unreachable": true}
fatal: [172.20.13.202]: UNREACHABLE! => {"changed": false, "msg": "Failed to connect to the host via ssh: ssh: Could not resolve hostname servera: nodename nor servname provided, or not known", "unreachable": true}
fatal: [172.20.13.203]: UNREACHABLE! => {"changed": false, "msg": "Failed to connect to the host via ssh: ssh: Could not resolve hostname servera: nodename nor servname provided, or not known", "unreachable": true}
Why synchronize can't resolve hostname 'servera'? In my host file, it's serverA.

Answer 1

serverA is the name of the group. There is no such host. (There might be more hosts in the group serverA. It would be difficult to decide which host delegate to.)
Try delegate to 172.20.5.121
- hosts: serverB
tasks:
- name: Copy Remote-To-Remote
remote_user: root
synchronize: src=/root/connection dest=/root/neutron-server.log
delegate_to: 172.20.5.121