Which statement best explains why the debt-to-GDP ratio is important?

a. It allows international comparisons of how indebted countries are.

b. If GDP is rising slower than debt, the government is, in some sense, living within its means.

c. It is a better measure than the level of debt because it takes into account a country's political stability.

d. A declining debt-to-GDP ratio raises concerns about a country's financial difficulties.