Concept:A vacuum has no particles, so it prevents heat transfer that requires a material medium.
Explanation:Conduction and convection both depend on particles to transfer heat energy.
In the space between the double walls of a thermos flask, the vacuum contains almost no particles.
Therefore, heat cannot be transferred across this space by conduction.
Convection currents also cannot form because there are no particles to move and carry heat.
Radiation, however, does not need a medium, so it can pass through a vacuum.
Radiation loss is mainly reduced by the silvered inner surfaces, not by the vacuum itself.
Thus, the vacuum is responsible for reducing heat loss through conduction and convection.
Answer:B. convection and conduction