Two cells of e.m.f. 10 V and 15 V are connected in parallel to each other between points A & B. The cell of e.m.f. 10 V is ideal but the cell of e.m.f. 15 V has internal resistance 1Ω. The equivalent e.m.f. between A and B is:
(a) 225 V
(b) not defined
(c) 15 V
(d) 10 V