Switchboard and Control panels

I have a study review for my California test and this questioned is asked: On switchboard and control panels exceeding 6' width, there shall be one entrance at each end of what dimension? a. 3' x 6' b. 18" x 7' c. 24" x 8' d. 24" x 6 1/2'. Now I have looked through Article 408, 409 and 110.26 to no avail... Is there an answer to this in the NEC or is the answer obvious? Thank you.