Refer to the below graph. Which of the following statements is correct?

The concept of "the invisible hand" suggests that

A) products are produced out of a seller's sense of charity. B) when the seller is better off, the buyer is worse off. C) sellers exploit consumers with high prices. D) buyers and sellers are self-interested. E) the command system is the only way of efficiently allocating resources.

A central concept in macroeconomics is the idea of the natural rate of unemployment

Why does it make sense to define full employment to occur when the unemployment rate equals the natural rate of unemployment, instead of when the unemployment rate equals zero? Elaborate and explain carefully.