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# Twelve cells, each having emf E volts are connected in series and are kept in a closed box. Some of these cells are wrongly connected with positive and negative terminals reversed. This 12 cell battery is connected in series with an ammeter , an external resistance R ohms and a two- cell battery (two cells of the same type used earlier , connected perfectly in series). The current in the circuit when the 12 - cell battery and 2-cell battery aid each other is 3A and is 2A when they oppose each other . Then, the number of cells in 12 -cell battery that are connected wrongly is :

$\begin {array} {1 1} (a)\;4 & \quad (b)\;3 \\ (c)\;2 & \quad (d)\;1 \end {array}$

(c) 2