Debt vs. GDP

I just heard on Bloomberg TV that the tipping point for an economy is the debt of a country reaching 90% of their GDP. We are quickly approaching this threshold.

I would like to know how much U.S. debt, China sold off. Yesterday on Bloomberg, China sold US bonds to a level that they are no longer the primary U.S. debt holder. China sold 34b dollars of U.S. debt.

Now on Bloomberg, they just said that Greece is at 12% of GDP. The U.S. is at 10% of GDP. Confusing, Very Confusing.

Define GDP using wiki: GDP = Consumption + Investment + Government Spending + (Exports - Imports)
OR
GDP=C+In+G+(E-I)

The sales of debt is good for America. No longer is China the largest debt holder of U.S. Debt. Japan is.