As a prefix ad- assumes the forms ac-, af-, ag-, al-, an-, ap-,
   ar-, as-, at-, assimilating the d with the first letter of the word to
   which ad- is prefixed. It remains unchanged before vowels, and before
   d, h, j, m, v. Examples: adduce, adhere, adjacent, admit, advent,
   accord, affect, aggregate, allude, annex, appear, etc. It becomes ac-
   before qu, as in acquiesce.