Two cells each of emf E and internal resistacne `r_(1)" and "r_(2)` respectively are conneted in series with an external resistance R. The potential defference between the terminats of the first cell will be zero when R equal to
A. `(r_(1)+r_(2))/2`
B. `sqrt(r_(1)^(2)r_(1)^(2))`
C. `r_(1)-r_(2)`
D. `(r_(1)r_(2))/(r_(1)+r_(2))`
