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

The image of a candle flame formed by a lens is obtained on a screen placed on the other side of the lens. If the image is three times the size of the flame and the distance between the lens and the image is $80 \, cm$,at what distance should the candle be placed from the lens? What is the nature of the image at a distance of $80 \, cm$ from the lens (in $, cm$)?

Zaheen uses a convex lens to make sunlight burn a hole in a piece of card. Zaheen changes the lens in the holder for another lens with twice the power. What will she have to do to focus the sunlight on the card?

If a convex lens forms a real and magnified image of an object,where is the object placed?

$(a)$ $A$ converging lens forms a real and inverted image of an object at a distance of $100 \, cm$ from it. Where should an object be placed in front of the lens so that the size of the image is twice the size of the object? Also, calculate the power of the lens.
$(b)$ State the laws of refraction.

An object $5\, cm$ high is kept $25\, cm$ away from a converging lens (convex lens) of focal length $10\, cm$. What is the nature,position,and size of the image?