Which of the following represents a system that is completely open?

a. a pressure cooker
b. a pressurized canister of helium gas
c. a capped soda bottle
d. a port of boiling water

The correct answer is D. The system that is completely open of the available answers would be an open pot of boiling water. This is due to the fact that an open system is defined as one in which energy or mass can be put out into or taken from the environment. Therefore, the evaporation of water due to the heat when a pot is boiling would be an example of lost energy, and therefore this is an open system.