Which of the following is not a rule to follow when adding an ending to a word?

a. double the final consonant if the vowel sound in the last syllable of the base word is short.
b. double the final consonant if the suffix to be added to the base word begins with a vowel.
c. double the final consonant if the last syllable of the base word is accented.
d. double the final consonant if the base word ends in "w" or "x."