When ions combine to form neutral compounds, their charges are usually crisscrossed to cancel out any net charge. For this example, Cr3+ and O2- have different charges. Applying the crisscross method, the neutral compound is Cr2O3. This proportion balances out a net charge because two Cr3+ gives a +6 charge, and three O2- gives -6 charge.
Here we assume that disintegration has occur so the normal t½ formular will not be used mathematically we..⅛×5600 =700 therefore it is about 700years option A
