Which of the following statements about including consulting engagements in the annual internal audit plan is true?

a. All requests for consulting engagements must be included in the annual internal audit plan, but there is no requirement to include consulting engagements

b. Assurance engagements must be included in the annual internal audit plan, but there is no requirement to include consulting engagements

c. Consulting engagements do not need to be included in the annual internal audit plan unless requested by the board

d. The acceptance of proposed consulting engagements into the annual internal audit plan may depend on their ability to add value