The reaction 2X→B is a zeroth order reaction. If the initial concentration of X is 0.2 M, the half-life is 6 h. When the initial concentration of X is 0.5 M, the time required to reach its final concentration of 0.2 M will be :-
(1) 18.0 h
(2) 7.2 h
(3) 9.0 h
(4) 12.0 h