A Question about shall, should and is

First of all, I know the difference of shall and should. No doubt of that. I just sometimes don't understand what is the degree of "is" in terms of requirement.
for example:
(A) The applicable speed limit, as established by law, is shown in multiples of 10 km/h.
(B) The applicable speed limit, as established by law, shall be in multiples of 10 km/h.
(C) The applicable speed limit, as established by law, should be in multiples of 10 km/h.
Here is the thing, (B) means absolutely, (C) means there is some exceptions, sometimes you can post it as 65 km/h if deemed necessary, so what is (A) mean?
closer to B or C?
Thank you.