here is a description of the sentence:
https://gss0.baidu.com/94o3dS.
it says above that all ports of HUB and SWITCH are in a broadcast domain, and each port on the router forms its own broadcast domain. I can understand the description of HUB and router, but all ports of SWITCH are in one broadcast domain
, because SWITCH divides VLAN, and each VLAN is a broadcast domain.