In the weak forms of the fluid flow model, as the weight functions (w1, w2) are virtual variations of the velocity components(vx, vy ),respectively, which relation is satisfied by the weight functions?

(a) \(\frac{\partial w1}{\partial x}+\frac{\partial w2}{\partial y}\)=0

(b) \(\frac{\partial w2}{\partial x}+\frac{\partial w1}{\partial y}\)=0

(c) \(\frac{\partial w1}{\partial x}-\frac{\partial w2}{\partial y}\)=0

(d) \(\frac{\partial w2}{\partial x}-\frac{\partial w1}{\partial y}\)=0

The question was posed to me at a job interview.

This key question is from Penalty in portion Flows of Viscous Incompressible Fluids of Finite Element Method