Which of the following is most appropriate unit to describe the rate at which a person reads?

Which of the following is most appropriate unit to describe the rate at which a person reads?

(A) minutes per word,because the independent quantity is the minutes.
(B) minutes per word,because the independent quantity is the words.
© words per minute,because the independent quantity is the minutes.
(D) words per minute, because the independent quantity is in words .