(B) The sign shall be mounted in such manner as to be legible to persons both in front of and behind the vehicle, and be mounted in the center of the vehicle between the warning lights.
(C) The sign shall consist of such material as to be stable against wind pressure and shall not consist of any such flexible material that would bend or reduce the visibility of the wording.
(D) The sign shall be at least seven inches in height, bearing the words "Caution - Children" in black letters on a yellow background. The letters shall be not less than four inches in height and not less than three-quarters of an inch in width of stroke.
(E) The sign shall be so constructed as to permit its removal from the vehicle when such vehicle is being used for purposes other than the transportation of pre-school children.