The viscosity terms in x-momentum equation is ( frac{ partial tau_{xx}}{ partial x}+ frac{ partial tau_{yx}}{ partial y}+ frac{ partial tau_{zx}}{ partial z} ). In a more general form, this becomes div( gradu). Which of these relations is used for this transformation?

• A. Thermodynamic relations
• B. Stress-strain relations
• C. Fluid flow relations
• D. Geometric relations