In which of the following pairs is the first situation an example of static equilibrium and the second an example of dynamic equilibrium?
a) A book resting on a table; CO2 dissolving and escaping from a sealed bottle of soda at constant pressure.
b) Two students pushing opposite ends of a stationary box with equal force; a decomposition reaction that has gone to completion with no products converting back.
c) Opposing traffic waiting at a red light; nitrogen and oxygen exchanging between air and water in a sealed aquarium with unchanging concentrations.
d) A pendulum hanging motionless at its lowest point; a block sliding at constant speed across an ice rink.