With a resistance '$X$' connected in series with a galvanometer of resistance $100\ \Omega$, it acts as a voltmeter of range 0 - 15 V. To double the range, a resistance of $1500\ \Omega$ is to be connected in series with '$X$'. The value of '$X$' in $\Omega$ is

The magnetic field at the center of circular coil carrying current '$I$' for a single turn of a given length of wire is '$B$'. The same wire is bent in a circular coil having two turns. When the same current passes through it, the value of magnetic field becomes

The magnetic field at perpendicular distance '$r$' from a long wire carrying current '$I$' is '$B$'. The magnetic field at perpendicular distance '$2r$' from the same wire is

Three coils of inductance, $L_1$ = 2 H, $L_2$ = 3 H and $L_3$ = 6 H are connected so that they are separated from each other. To obtain the effective inductance of $\dfrac{18}{11}$ H between points A and B, out of the following figures, the correct one is