The equation for the vibration of a string fixed at both ends vibrating in its third harmonic is given by $\mathrm{y}\mathrm{\hspace{0.33em}}=\mathrm{\hspace{0.33em}}2 cm\mathrm{\hspace{0.33em}}\sin \left[{\left(0.6\mathrm{cm}\right)}^{-1}\mathrm{\hspace{0.33em}}\mathrm{x}\right]\mathrm{\hspace{0.33em}}\cos \left[\left(500{\mathrm{\pi s}}^{-1}\right)\mathrm{\hspace{0.33em}}\mathrm{t}\right]\mathrm{\hspace{0.33em}}$The length of the string is :

• A

  24.6 cm

• B

  12.5 cm

• C

  20.6 cm

• D

  15.7 cm