Consider a cylinder of height h and radius a, closed at both ends, centred at the origin. Let \[ ix + jy + kz\] be the position vector and \[ n\] a unit vector normal to the surface. The surface integral \[ _S r n\, dS\] over the closed surface-of the cylinder is

Consider a cylinder of height h and radius a, closed at both ends, centred at the origin. Let $$\hat{i}x+\hat{j}y+\hat{k}z$$   be the position vector and $$\hat{n}$$ a unit vector normal to the surface. The surface integral $$\underset{S}{\int }\overrightarrow{r}\cdot \hat{n}dS$$   over the closed surface-of the cylinder is

Answer: Option B