Several cells are linked in series, as illustrated. What is the total electromotive force (e.m.f.) of the cells?
- Athe average of the e.m.f.s of the separate cells
- Bthe e.m.f. of one of the cells
- Cthe product of the e.m.f.s of the cells
- Dthe sum of the e.m.f.s of the cells