Which One of the Following Situations Is a Direct Application

Which one of the following situations is a direct application of the Zeroth Law of Thermodynamics?

A) Block A has twice the temperature of block B before they are brought into contact. Upon contact, heat flows from block A to block B.
B) A sample of gas within a cylinder with a piston is held at constant temperature and pressure while it is allowed to expand. During this process, the gas absorbs heat from its surroundings.
C) The motor of a refrigerator uses electric energy to remove heat from inside the refrigerator and transfer it to the room.
D) A physicist removes energy from a system in her laboratory until it reaches a temperature of 3 × 10^-10 K, a temperature very close to (but still greater than) absolute zero.
E) A thermometer is calibrated by placing it in an ice water bath within an adiabatic container until the thermometer is in thermal equilibrium with the ice water.