The Coefficient of Multiple Determination (R²),with Two Independent Variables,is

The coefficient of multiple determination (R²) ,with two independent variables,is

A) the amount of variation explained by X₁ plus the amount of variation explained by X₂ after controlling for the effect of X_1.
B) the amount of variation explained by X₁ minus the amount of variation unexplained by X_2.
C) the total variation in Y minus the amount unexplained by X₁ after controlling for X_2.
D) the sum of all explained and unexplained variation in X₁ and X_2.