A liquid remains hot or cold for a long time in a thermos flask because there is no loss or gain of heat by

Conduction
Convection and radiation
Both (a) and (b)
None of the above

సమాధానం చూపించు & వివరణ

సమాధానం: (C) Both (a) and (b)

వివరణ

A vacuum flask's double wall with a vacuum stops conduction and convection, and the silvered surfaces cut radiation. So heat is not gained or lost by either (a) or (b).