If, during 2010, the Republic of Sildavia recorded investment spending of $3 billion, government purchases of $3 billion, consumer spending of $7 billion, imports of $5 billion, government transfers of $1 billion, and exports of $3 billion, Sildavia's GDP in 2010 was:
a. $11 billion.
b. $12 billion.
c. $13 billion.
d. $14 billion.