Two cells x and y of identical emf but different internal resistance `r_(1) and r_(2) (r_(1) gt r_(2))` are connected in series to an external resistance R. If the terminal voltage across x is zero, then

Options

A. `R= r_(1) + r_(2)`
B. `R= r_(1)-r_(2)`
C. `R= sqrt(r_(1)r_(2))`
D. `R= (r_(1) + r_(2))/(2)`