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(μ gradu). Which of these relations is used for this transformation?

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