If a country's debt-to-gdp ratio is currently 5% and its debt is expected to grow from 20 billion dollars to 40 billion in the next 25 years, what will be the country's gdp have to be in 25 years to maintain the current debt-to-gdp ratio?
a. 400 billion dollars
b. 2 billion dollars
c. 800 billion dollars
d. 1 billion dollars