Two cells of equal e.m.f. and of internal resistances r1 and r2 (r1 > r2) are connected in series. On connected in series. On connecting this combination to an external resistance R, it is observed that the potential difference across the first cell becomes zero. The value of R will be
(1) r1 + r2
(2) r1 - r2
(3) r1 + r2/2
(4) r1 - r2/2
