For an Open Economy, the Equation Y = C

Multiple Choice

For an open economy, the equation Y = C + I + G + NX is an identity. If we define national saving, S, as the total income in the economy that is left after paying for consumption and government purchases, then for an open economy, it is true that

A) S = I.
B) S = 0.
C) I = S + NX.
D) S = I + NX.