In the Equation GDP = C + I + G

In the equation GDP = C + I + G + F, in which F equals net export spending i.e., total spending on exports minus total spending on imports) , imports are subtracted from the other types of expenditures because:

A) imports reduce national welfare.
B) other countries do not import goods from the U.S.
C) the value of imports is included in the other components of spending such as consumption spending.
D) the value of imports is difficult to determine due to the fact that they are frequently stated in terms of foreign currency.