正解:A
In the scenario provided, both switches SW1 and SW2 are set to "dynamic auto" mode for their respective interfaces. This mode allows the interface to become a trunk link if the connecting interface is set to trunk or desirable mode. However, when both ends of a link are set to "dynamic auto," neither will actively attempt to form a trunk. To resolve this issue, one of the switches needs to actively attempt to form a trunk, which can be achieved by setting one of the interfaces to "dynamic desirable" mode using the command switchport mode dynamic desirable.
References:
* Implementing and Operating Cisco Service Provider Network Core Technologies (SPCOR) v1.1
* Cisco's training and certification materials regarding DTP (Dynamic Trunking Protocol) and switchport modes.