What are the uses of the $Ni-Cd$ cell?

Saturated solution of $KNO_3$ is used to make salt bridge because

$A$ hypothetical electrochemical cell is shown below $A | A^{+} (xM) || B^{+} (yM) | B$. If the cell potential is $+0.20 \ V$,the cell reaction is :-

Match List-$I$ with List-$II$:

List-$I$ (Applications)	List-$II$ (Batteries/Cell)
$A$. Transistors	$I$. Anode $- Zn/Hg$; Cathode $- HgO + C$
$B$. Hearing aids	$II$. Hydrogen fuel cell
$C$. Inverters	$III$. Anode $- Zn$; Cathode $- Carbon$
$D$. Apollo space ship	$IV$. Anode $- Pb$; Cathode $- PbO_2$

Choose the correct answer from the options given below:

In animals,nerve signals are generated due to the movement of which of the following?

The following four solutions are kept in separate beakers and copper metal is put in each of them. Which solution will become blue after some time?