Which statement is correct:

1. Iron rims are cooled before they are placed on cart wheels.
2. A glass stopper gets tight on warming the neck of the bottle.
3. Telephone wires sag in winter, but becomes tight in summer.
4. A little space is left between two rails on a railway track.

Heat in a liquid is transferred by:

1. conduction
2. convection
3. radiation
4. conduction and radiation

The vacuum kept in between the walls of a thermos flask reduces the heat transfer by:

1. conduction only
2. convection only
3. radiation only
4. conduction and convection

What is heat? State its S.I. unit.