$A$ conducting sphere of radius $R$ is given a charge $Q$. The electric potential and the electric field at the centre of the sphere respectively are

The concentric spheres of radii $R$ and $r$ have positive charges $q_{1}$ and $q_{2}$ with equal surface charge densities. What is the electric potential at their common centre?

Given below are two statements $:$ one is labelled as Assertion $A$ and the other is labelled as Reason $R$.
Assertion $A :$ Work done in moving a test charge between two points inside a uniformly charged spherical shell is zero,no matter which path is chosen.
Reason $R :$ Electrostatic potential inside a uniformly charged spherical shell is constant and is same as that on the surface of the shell.
In the light of the above statements,choose the correct answer from the options given below.

Two charges of $4\,\mu C$ each are placed at the corners $A$ and $B$ of an equilateral triangle of side length $0.2\, m$ in air. Find the electric potential at $C$. $\left[ {\frac{1}{{4\pi {\varepsilon _0}}} = 9 \times {{10}^9}\,\text{N m}^2/\text{C}^2} \right]$

Two positive charges $Q$ and $4Q$ are placed at points $A$ and $B$ respectively,where $B$ is at a distance $d$ units to the right of $A$. The total electric potential due to these charges is minimum at point $P$ on the line segment joining $A$ and $B$. What is the distance of $P$ from $A$?

Two isolated,concentric,conducting spherical shells have radii $R$ and $2R$ and uniform charges $q$ and $2q$ respectively. If $V_1$ and $V_2$ are potentials at points located at distances $3R$ and $\frac{R}{2}$,respectively,from the centre of the shells,then the ratio $\left(\frac{V_2}{V_1}\right)$ will be: