Physics
Stationary wave is formed by

A transverse wave superposing a longitudinal wave
Two waves of same frequency travelling in the opposite direction
Two waves of the same speed superposing
Two waves of same frequency travelling in the same direction

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

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

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