Keep Smiling Dental Surgery Is Located in Christchurch

Keep Smiling Dental Surgery is located in Christchurch. The company employs three dentists, four dental assistants and two receptionists who book appointments and keep records. The surgery provides a range of services including general dental care, paediatric dental care and teeth whitening. The clinic includes a small retail section that carries specialised products for home treatment. The surgery has recently updated its communications plan and negotiated a plan with a new provider which includes a monthly line charge and a usage charge for internet activity. The cost structure for communications is best described as a:

A) Fixed cost
B) Marginal cost
C) Sunk cost
D) Mixed cost