LWratio decidendi

This is a question and its answer from acca study hub. I understood why the option D is correct answer but i did not understood why option B is incorrect? Isn't the ratio decidendi binding on future decisions? I have copied the question and answer from study hub below. Which of the following statements regarding the doctrine of precedent is correct? a) Obiter dicta is binding on future decisions b) The ratio decidendi is binding on future decisions c) If a precedent is overruled, the earlier decision is nullified d) The ratio decidendi can be subsequently overruled in certain cases The correct answer is D. It is the ratio decidendi that is the precedent (not obiter dicta). Precedent is not absolute because it may be overruled in another case decided at a higher level. In this case the earlier judgement is not changed; the precedent is set aside and will not be followed in future. (An earlier decision is nullified if a decision is reversed by a higher court.)