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.What is the meaning of "ad hoc" in this instance?

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 rather than as permanent employees.