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