A wire is stretched to double its length .what happened to its resistance and resistivity?

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Resistivity of wire remains constant. As the length of wire gets doubled, the cross-sectional area will become half of its previous value because volume of wire remains constant. Hence, we can see that the new resistance is four times the previous resistance.

The best answer i can think of is b

Sounds propagates through some kind of medium. usually it's air, but it goes just as well through solid objects like walls, windows etc. a soundwave will compress the (air)molecules in front of it, and if this increased density wave hits your ears, you register it as sound. solid objects like walls are a lot denser than air, this means that they will damp a wave, but also carry it easier.