A convex lens forms a real image $9 \mathrm{cm}$ long (high) on a screen. Without altering the position of the object and the screen, the lens is displaced and we get again a real image $4 \mathrm{cm}$ long(high) on the screen. Then the length of the object is-

In the figure given below, there are two convex lens $L_1$ and $L_2$ having a focal length of $f_1$ and $f_2$ respectively. The distance between $L_1$ and $L_2$ will be

What should be the value of distance $d$ so that the final image is formed on the object itself. (focal lengths of the lenses are written on the lenses).