Refer to the exhibit. Users from the Engineering VLAN complain that every time Business VLAN users have a network connectivity issue， the Engineering VLAN users usually have problems experiencing slow response or network connectivity problems. After troubleshooting， an unauthorized switch 2 was found. This unauthorized switch has been a regular problem， assuming the root bridge function under the spanning-tree domain and causing the Engineering VLAN to be unstable. Which three of these actions could be suggested to fix the problem？（）

A.Upgrade Spanning Tree Protocol to Rapid Spanning Tree Protocol.
B.Change Business VLAN PCs to switch 1 and switch 4.
C.Force the root bridge to be switch 2， instead.
D.Adjust spanning-tree timers （max-age and forward-delay）.
E.Shut down all unused ports.
F.Use MSTP to separate the Engineering VLAN from the Business VLAN to optimize spanning-tree convergence time within each VLAN.