Physics
In superconductors, the energy gap is due to

Electron-electron interaction
Electron-phonon interaction
Electron-photon intercation
Phonon-phonon interaction

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Physics
Water pipes in hilly areas often burst on a cold frosty night, because

The material of which pipes are made contracts due to cold and so breaks
Water in the pipes freezes and on freezing water expands, so pipes break
Frost makes the pipes rusty, so they break
None of these

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