Which two of the given transverse waves will give stationary waves when get superimposed?

${\mathrm{z}}_1 =\mathrm{acos}(\mathrm{kx}-\mathrm{\omega t}) \left(\mathrm{A}\right)$

${\mathrm{z}}_2 = \mathrm{acos}(\mathrm{kx}+\mathrm{\omega t}) \left(\mathrm{B}\right)$

${\mathrm{z}}_3 = \mathrm{a} \cos (\mathrm{ky}-\mathrm{\omega t}) \left(\mathrm{C}\right)$

• A

  A and B

• B

  A and C

• C

  B and C

• D

  Any two