Which one of the following materials cannot be used to make a lens?

$A$ spherical mirror and a thin spherical lens have each a focal length of $-15\,cm$. The mirror and the lens are likely to be

An object is placed at a distance of $10\, cm$ from a convex mirror of focal length $15\, cm$. Find the position and nature of the image.

Where should an object be placed in front of a convex lens to get a real image of the size of the object?

$A$ convex lens forms a real and inverted image of a needle at a distance of $50\, cm$ from it. Where is the needle placed in front of the convex lens if the image is equal to the size of the object? Also,find the power of the lens.

No matter how far you stand from a mirror,your image appears erect. The mirror is likely to be