Final answer:
GTB Computers is the best example of a mixed marketing channel as they use their own sales force and external local dealers to provide products and after-sales services.
Step-by-step explanation:
The best example of a mixed marketing channel is option A) GTB Computers. They use their own sales force to initiate sales contacts, but also rely on external local dealers to provide products and after-sales services. This approach allows GTB Computers to leverage the expertise and reach of the external dealers while maintaining control over their sales process through their own sales force.