$13.$ One source of competitive advantage is 'strategic relationships'. If you were running a well$-$known Advertising Agency, select the ONE option in the list below relating to the agency's relationships that you would consider as offering a sustainable competitive advantage.

A$.$ Client business is built into fixed term contracts of one or two years

B$.$ Extensive selection of testimonials from satisfied customers from a diverse range of industries

C$.$ The only parent company in the agency world whose policy is to invest centrally in building capabilities it then shares across business units

D$.$ An award$-$winning creative team