Two cells of same emf but different internal resistance \( r_1 \) and \( r_2 \) are connected in series with a resistance \( R \). The value of resistance \( R \), for which the potential difference across second cell is zero, is:
\( r_2 - r_1 \)
\( r_1 - r_2 \)
\( r_1 \)
\( r_2 \)