The output of the show aaa domain-map displays the following information:
Domain: isp1.com; virtual-router: vr1; loopback: 1
Domain: isp2.com; virtual-router: vr2; loopback: 1
Domain: none; virtual-router: vr3; loopback: 1
Domain: default; virtual-router: vr4; loopback: 1
The end user supplies a login name of joe with no domain name.
Which virtual router is responsible for authenticating this user?（）