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

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

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

     ${\mathrm{z}}_3=\mathrm{acos}(\mathrm{ky}-\mathrm{\omega }\mathrm{t})$      .....(C)

• A

  A and B

• B

  A and C 

• C

  B and C

• D

  Any two