If the velocity gradient is given by $$\theta$$ and dynamic viscosity of the fluid is given by $$\upsilon$$. What is the shear stress on the wall of the boundary layer in the direction of motion?

A

$$ \mu \theta $$ 

B

$$ \mu  + \theta $$

C

$$ \frac {\mu} {\theta} $$

D

$$ \frac {\theta} {\mu}$$