Answer
Choice C.
Work Step by Step
The induced emf is given by Faraday’s Law.
$$\epsilon=-\frac{d\Phi_B}{dt}= -A\frac{dB_{av}}{dt}$$
The induced current is the emf divided by the resistance, so it too is proportional to the rate at which the magnetic field is changing.
In summary, the current is proportional to the slope of the B-t graph.
Study the graph in the introduction to the passage problems. The magnetic field increases rapidly (positive slope), reaching a maximum value at around 0.15 ms (zero slope), and then gradually decreases (slight negative slope) to zero.
Therefore, the current is initially positive, then becomes zero when the B-t graph is a maximum, at about 0.15 ms. It then becomes negative as the slope becomes negative, and gradually approaches zero. This most resembles graph C.
