The following surface integral is to be evaluated over a sphere for the given steady velocity vector field $F$ = $xi$ + $yj$+ $zk$ defined with respect to a Cartesian coordinate system having $i$, $j$ and $k$ as unit base vectors.

$\iint_{s}^{ }\frac{1}{4}(F.n)dA$

where $S$ is the sphere, $x^2$ + $y^2$ + $z^2$ =$1$ and n is the outward unit normal vector to the sphere. The

value of the surface integral is

- $\pi$
- $2\pi$
- $3\pi/4$
- $4\pi$