Puff Animal Hospital Looks After the Health of Household Pets

Puff Animal Hospital looks after the health of household pets such as dogs, cats, birds and rabbits. The hospital wants to use an activity based costing system to allocate costs to each animal case it handles. These cases tend to vary in their complexity: some pets are here for a regular check ups and immunization; other pets have emergencies that require surgery. Some pets are very exotic and require the attention of the supervisor vet, others are common animals with common problems such that only the junior vets are needed. Puff Animal Hospital is quite large; on average there are five vets, a general assistant, and two receptionists on duty at one time.
Which of the following is an example of a facility sustaining activity for Puff Animal Hospital?

A) The supervisor vet supervises junior vets in some of the more complex cases.
B) Processing invoices.
C) The general vet gives health checks for small dogs.
D) The general assistant cleans the hospital.