ABC Bank utilizes the janitorial services of Bob's Scrub-A-Dub-Dub to keep their buildings clean. Bob's provides all the necessary cleaning supplies, training of crews, background security checks (since the crews work after hours) and all other associated expenses. The other available janitorial services do not typically provide security checks and a client such as a bank would have to hire guards to watch the work of the janitors. This expense effectively limits the bank to utilizing only Bob's services. This illustrates the concept of

A. high employment costs.
B. unfavorable supplier status.
C. high switching costs.
D. favorable quality status.
E. complementary products