Two forks $\mathrm{A}$ and $\mathrm{B}$ when sounded together produce 4 beats $/ \mathrm{s}$. The fork A is in unison with $30 \mathrm{~cm}$ length of a sonometer wire and $B$ is in unison with $25 \mathrm{~cm}$ length of the same wire at the same tension. The frequencies of the fork are
(A) $24 \mathrm{~Hz}, 28 \mathrm{~Hz}$
(B) $20 \mathrm{~Hz}, 24 \mathrm{~Hz}$
(C) $16 \mathrm{~Hz}, 20 \mathrm{~Hz}$
(D) $26 \mathrm{~Hz}, 30 \mathrm{~Hz}$