Selim has been out in cold weather and his hands feel frozen. He holds a cup of hot tea in his hands. What kind of heat transfer is he using to increase the thermal energy in his hands?

A conduction
B convection
C radiation
D combustion

A. conduction  

Explanation:

Thermal energy in the hot tea is transferred to the material of the cup by direct contact.

The cup in turn transfers its thermal energy to their cold hands by direct contact.

The transfer of thermal energy by direct contact is conduction.

B is wrong. Convection is the movement of the hotter particles within a fluid — like water or or air — in which the warmer portions rise and the cooler portions sink.

C is wrong. Radiation can transfer energy through a vacuum. It does not need a medium to carry the energy.

D is wrong. Combustion involves  the burning of something.

the reaction mixture contains some ammonia, plus a lot of unreacted nitrogen and hydrogen. the mixture is cooled and compressed, causing the ammonia gas to condense into a liquid. the liquefied ammonia is separated and removed. the unreacted nitrogen and hydrogen are then recycled back into the reactor .