In a Contract for Services,a Key Personnel Clause

In a contract for services,a key personnel clause:

A) legally cannot be included because work assignment is the responsibility of the service provider.
B) is undesirable from the buyer's perspective because on-going negotiations are a better way to ensure the best personnel are assigned throughout the contract.
C) should always be included no matter the nature of the service.
D) may be included if success depends on specific individuals.
E) is desirable from the supplier's perspective to ensure that the buyer cannot demand personnel changes mid-contract.