The Sustainable Upper Limit of Real GDP Is a Level

The sustainable upper limit of real GDP is a level of GDP that is

A) potential GDP.
B) greater than potential GDP, but by how much greater is unknown and controversial.
C) less than potential GDP, but by how much less is unknown and controversial.
D) determined only by what is the full employment equilibrium in the labor market.
E) None of the above answers is correct because there is no sustainable upper limit to real GDP because real GDP can always be increased.