Consider a binary solution of A and $B$ with the concentration of $x_A=0\mathrm{.}3.$ The free energy of this binary solution is given by $G=G^0+RT(x_A{x}_B).$ Suppose the trace diffusion coefficient of $A$ is triple that of $B,$ i$.$ e$.,D_A=3D_B,$ Then, what is the interdiffusion coefficient, widetilde$(D)?($Express your answer in term of $D_B)$