In the following questions do not perform any calculations. You may, if you like, include sketches: c. Explain why for the cylindrical surface x² + y² = 1, -15 zs1 if xds=0; ii.ff, (i+j). ds = 0; iii.ff, (i+j). ds = 4x clue: in part (iii) consider the form of the unit normal to this surface in caartesian coordinates