Physics
When a piece of pure silicon is doped with aluminium, then

The resistivity of the doped silicon piece will increase
The conductivity of the doped silicon piece will remain the same
The doped silicon piece will become n-type
The doped silicon piece will become p-type

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Physics
Rainbow is due to

absorption of sunlight in minute water droplets
ionisation of water deposits
diffusion of sunlight through water droplets
refraction and reflection of sunlight by water droplets

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