The displacement of a particle in string stretched in X direction is represented by y. Among the following expressions for y, those describing  wave motions are

• A

  $\mathrm{coskxsin\omega }\mathrm{t}$

• B

  ${\mathrm{k}}^2{\mathrm{x}}^2-{\mathrm{\omega }}^2{\mathrm{t}}^2$

• C

  $\cos (\mathrm{kx}+\mathrm{\omega }\mathrm{t})$

• D

  $\cos ({\mathrm{k}}^2{\mathrm{x}}^2-{\mathrm{\omega }}^2{\mathrm{t}}^2)$