What is necessary for a convection cell to be set up in a fluid? a) changes in density b) thermal gradient c) thermal gradient and changes in the density

Convection cells don't work in solids.

In order for convection to happen, material with some temperature and density has to FLOW to a place where the material there has a different temperature and density.

Material doesn't FLOW in solids, only in FLuids.

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it's when something is going so fast it makes a loud noise, breaking the sound barrier, for example: a jet makes a loud noise, not only cause of the jet but because they reached a point where they went so fast that you can year the noise from various parts, kinda like a police car, lets say the car was going   so fast that you hear the siren of the cop car right next to you, but the car is probably a mile a way, it's cos the siren was next to you a few seconds if you understand me