The office of a member of the Council shall become vacant—
(a) if he or she ceases to be a member of the Trust under rules 19 or 20;
(b) if he or she is absent from 3 consecutive meetings of the Council without reasonable excuse;
(c) if he or she gives the Council one calendar month's notice in writing that he or she resigns his or her office;
(d) if he or she is removed by an extraordinary resolution passed at a general meeting of the Trust.