Enercare is an Ontario energy company, known for submetering houses and subdivisions for billing. They've been servicing the Toronto area since 1848. Their years of experience as well as relationships with every reputable builder in Ontario has afforded them the luxury of being the preferred vendor of many municipalities. Enercare has gained what type of competitive advantage?

a) sustainable competitive advantage

b) product differentiation competitive advantage

c) niche competitive advantage

d) cost competitive advantage