A square conducting loop of side $a$ moves to the right at a constant speed $v$. It passes through a region of width $2a$ in which there is a uniform magnetic field into the page, as shown in the figure. The loop enters the field at $t = 0$. Take the emf as positive when the induced current is anticlockwise. Which graph shows the induced emf $\varepsilon$ against time, from $t = 0$ to $t = 3a/v$?

Fig. 1: A square loop of side a moving to the right at speed v towards a region of width 2a that has a uniform magnetic field into the page.

Graph (a)Graph (b)Graph (c)Graph (d)