When Consumers Change Mobile Phone and Data Service Providers, They

When consumers change mobile phone and data service providers, they may be required to maintain service with the provider for a specified time period.  This is an example of a: 

A) cost to a producer to exchange equipment in a facility when new technologies emerge.
B) cost of changing the firm's strategic group.
C) one-time cost suppliers incur when selling to a different customer.
D) one-time cost customers incur when buying from a different supplier.