A dimer, X, remains stable in the solid state, whereas in solution a dynamic equilibrium is established. $\text{X(aq)} \rightleftharpoons 2\text{Y(aq)}$ A solution of X initially has a concentration of $0.50\,\text{mol dm}^{-3}$. Once equilibrium is attained, $[\text{X(aq)}]$ has decreased to $0.25\,\text{mol dm}^{-3}$. The way in which $[\text{X(aq)}]$ and $[\text{Y(aq)}]$ vary with time is shown on a graph until equilibrium is reached. The value of $K_c$ is then worked out. Which graph and $K_c$ value are correct?