Consider a closed surface $S$ surrounding a volume V. If $\vec{r}$ is the position vector of a point inside $S$, with $\hat{n}$ the unit normal on $S$, the value of the integral $\unicode{x222F}_S \; 5 \vec{r} . \hat{n} d S$ is
- $3 \mathrm{V}$
- $5 \mathrm{V}$
- $10 \mathrm{V}$
- $15 \mathrm{V}$