Consider the three waves ${\mathrm{z}}_1, {\mathrm{z}}_2 \mathrm{and} {\mathrm{z}}_3$ as
 ${\mathrm{z}}_1=\mathrm{Asin}(\mathrm{kx}-\mathrm{\omega }\mathrm{t})$, ${\mathrm{z}}_2=\mathrm{Asin}(\mathrm{kx}+\mathrm{\omega }\mathrm{t})$
and  ${\mathrm{z}}_3=\mathrm{Asin}(\mathrm{ky}-\mathrm{\omega }\mathrm{t})$. Which of the following represents a standing wave 
 

• A

  ${\mathrm{z}}_1+{\mathrm{z}}_2$

• B

  ${\mathrm{z}}_2+{\mathrm{z}}_3$

• C

  ${\mathrm{z}}_3+{\mathrm{z}}_1$

• D

  ${\mathrm{z}}_1+{\mathrm{z}}_2+{\mathrm{z}}_3$