Valdyn,Jacob,and Bob Have Just Finished a Review of Their Business's

Valdyn,Jacob,and Bob have just finished a review of their business's legal expenses for the past year.While there has been an increase,it hasn't been substantial enough to warrant changing their ad hoc retainer relationships with either Sheps & Shaps LLP or Dunn & Wright LLP.In this instance,what is the meaning of "ad hoc"?

A) The professional responsibilities of each firm are identical.
B) The firms receive annual retainers.
C) The relationships are regularly reviewed for cost-effectiveness.
D) The firms are hired on a project basis.