Using the Prandtl's mixing length concept, how is the turbulent shear stress expressed?

• A. \[\rho l\,\overline{\frac{du}{dy}}\]                      
• B. \[\rho {{l}^{2}}\,\overline{\frac{du}{dy}}\]
• C. \[\rho l\,{{\left( \overline{\frac{du}{dy}} \right)}^{2}}\]               
• D. \[\rho {{l}^{2}}\,{{\left( \overline{\frac{du}{dy}} \right)}^{2}}\]