急问一道04年的语法题

flairUID: 2173190

04年8月14题
A lingua franca is any auxiliary language, -------a rudimentary kind, used as a medium of communication between people who speak different languages.
(A) of which usually
  (B) which usually of
  (C) is usually of
  (D) usually of
答案选D，为什么，of 前能用usually吗？