Which of the Following Is NOT Given as a Source

Which of the following is NOT given as a source of customer-introduced variability to a service delivery system?

A) Uncertainty in what customers will prefer from the service product bundle.
B) Uncertainty in the ability of customers to participate in the service.
C) Uncertainty in the customer's willingness to interact with back-office service providers.
D) All of the above are sources of customer-introduced variability.