What Does Okun's Law Refer To

What does Okun's law refer to?

A) The relationship between job vacancies and unemployment.
B) The seemingly constant ratio between those in the labour force and the total population.
C) The relationship between the level of cyclical unemployment and the difference between potential and actual GDP.
D) The difference between an economy's real and nominal GDP.
E) The time it takes a number to double given a certain percentage growth rate.