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

1. A

   Conduction

   (a) Conduction. Partly true: the vacuum jacket blocks conduction through air, but radiation would still leak without silvering, and convection is also blocked. Conduction alone is incomplete.

2. B

   Convection and radiation

   (b) Convection and radiation. Also incomplete if conduction is omitted; the flask is designed against all three.

3. C

   Both (a) and (b)

   (c) Both (a) and (b). A thermos uses vacuum (no conduction or convection through gas) plus silvered walls (cuts radiation). Heat is therefore not lost or gained by conduction, convection or radiation. That combined answer is the official key.

4. D

   None of the above

   (d) None of the above. The combined mechanism is listed in (c), so ‘none’ fails.

Summary. Official key is (c) both (a) and (b). Thermos: vacuum stops conduction and convection; silvering stops radiation. Honour the stored letter (c).