The internal resistance of a cell depends on

Three identical cells,each of emf $10 \ V$ and internal resistance $3 \ \Omega$,are connected in parallel across terminals $A$ and $B$. The net emf across $A$ and $B$ is .......... $V$.

The potential difference between the terminals of a cell is $20 \ V$ when a current of $2 \ A$ flows through the circuit. When the direction of current in the circuit is reversed,the potential difference between the terminals of the cell is $30 \ V$. The internal resistance of the cell is (in $Omega$)

Given below are two statements $:$
Statement-$I$ : The equivalent emf of two non-ideal batteries connected in parallel is smaller than either of the two emfs.
Statement-$II$ : The equivalent internal resistance of two non-ideal batteries connected in parallel is smaller than the internal resistance of either of the two batteries.
In the light of the above statements,choose the correct answer from the options given below.

$A$ battery consists of a variable number '$n$' of identical cells,each having an electromotive force '$E$' and internal resistance '$r$',connected in series. The terminals of the battery are short-circuited,and the current '$i$' is measured. Which of the graphs below shows the relationship between '$i$' and '$n$?'

Four cells,each of emf $E$ and internal resistance $r$,are connected in series across an external resistance $R$. By mistake,one of the cells is connected in reverse. Then the current in the external circuit is