The equivalent conductance of $\frac {M}{32}$ solution of a weak monobasic acid is 8.0 mho $cm ^2$ and at infinite dilution is 400 mho $cm ^2$, The dissociation constant of this acid is -

Question

( A ) $1.25\times10^{-4}$
( B ) $6.25\times10^{-4}$
( C ) $1.25\times10^{-6}$
( D ) $1.25\times10^{-5}$