The electric potential $V$ of a charged spherical shell depends on the distance $r$ from its center. Which of the following graphs represents this dependence?

$A$ charge of $10 \,\mu C$ is placed at the origin of an $x-y$ coordinate system. The potential difference between two points $(0, a)$ and $(a, 0)$ in volts will be

Two electric charges of $9 \mu C$ and $-3 \mu C$ are placed $0.16 \ m$ apart in air. There will be a point $P$ on the line joining the two charges and between them where the electric potential is zero. The distance of $P$ from the $9 \mu C$ charge is: (in $m$)

Four equal point charges $Q$ are placed in the $xy$ plane at $(0, 2), (4, 2), (4, -2)$ and $(0, -2)$. The work required to put a fifth charge $Q$ at the origin of the coordinate system will be

Electric potential at a point distant $0.5 \,m$ from a spherical conductor of radius $0.2 \,m$ charged to $+1 \,nC$ is

The radius of a soap bubble whose potential is $16\,V$ is doubled. The new potential of the bubble will be.....$V$