The working of a dynamo is based on the principle of

Two circular coils $A$ and $B$ are facing each other as shown in the figure. The current $i$ through coil $A$ can be altered.

Assertion $(A)$: When a circular coil,placed in a region with its plane parallel to a magnetic field,expands radially outwards,no emf is induced in it.
Reason $(R)$: There is a constant magnetic field in the perpendicular (to the plane of the coil) direction.

The magnetic field of an electromagnetic wave in a certain region obeys the relation $B = 10^{-12} \sin(5 \times 10^6 t) \text{ T}$,where $t$ is the time. Then,the induced emf in a coil of $300$ turns and area $20 \text{ cm}^2$,oriented perpendicular to the field,is:

$A$ conducting loop of radius $\frac{10}{\sqrt{\pi}}\,cm$ is placed perpendicular to a uniform magnetic field of $0.5\,T$. The magnetic field is decreased to $zero$ in $0.5\,s$ at a steady rate. The induced emf in the circular loop at $0.25\,s$ is:

$A$ circular coil has $100$ turns,radius $3 \ cm$ and resistance $4 \Omega$. This coil is co-axial with a solenoid of $200$ turns/cm and diameter $4 \ cm$. If the solenoid current is decreased from $2 \ A$ to zero in $0.04 \ s$,then the current induced in the coil is