3 At one particular temperature, the reaction AB has a value of Kc = 4.2 x 10-4. What is the equilibrium concentration of B-if the reaction starts with only 0.5 M of A present? O 2.1 x 104 M O 1.2 x 10-4 M 6.5 x 10-4 M 4.2 x 10-4 M N