【单选题】
A switch that is to be added to the production network has been preconfigured （trunks， VLANs， VTP， and STP） and was tested in your lab. After installing the switch into the network， the entire network went down. <br /> What might explain what happened？ （）
A、The new switch happened to be running Cisco Catalyst operating system， while the other network switches were running Cisco IOS Software.
B、The configuration revision of the new switch was higher than the configuration revision of the production VTP domain.
C、The link costs on the new switch are set to a high value， causing all ports on the new switch to go into a forwarding mode and none into blocking mode， thereby causing a spanning-tree loop.
D、The ports connecting to the two switches have been configured incorrectly. One side has the command switchport mode access and the other switchport mode trunk.
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