For $0.05\mathrm{M}$ of a weak monobasic acid, the molar conductivity is $7.8\mathrm{S}\mathrm{c}{\mathrm{m}}^2\mathrm{mo}{\mathrm{l}}^{-1}$ and the limiting molar conductivity is $390\mathrm{S}\mathrm{c}{\mathrm{m}}^2\mathrm{mo}{\mathrm{l}}^{-1}$. Taking $\alpha ={\Lambda }_m/{\Lambda }_m^{\circ }$ and $K_a=c{\alpha }^2$, $K_a$ is

1. A$2.0\times 10^{-5}$
2. B$1.0\times 10^{-3}$
3. C$4.0\times 10^{-5}$
4. D$1.0\times 10^{-5}$