# A d.c. main supply of e.m.f. 220 V is connected across a storage battery of e.m.f. 200 V through a resistance of $\;\Omega\;$. The battery terminals are connected to an external resistance ' R ' . The minimum value of ' R ' , so that a current passes through the battery to charge it is :

$(a)\;7\;\Omega \qquad(b)\;9\;\Omega\qquad(c)\;11\;\Omega\qquad(d)\;zero$