Louie made 8.1 liters of fruit punch using only grape juice and carbonated water. He used 4.2 liters of grape juice. The equation 4.2+c= 8.1 can be used to represent this situation, where c is the number of liters of carbonated water used. How many liters of carbonated water did Louie use?
a.)3.9 liters
b.)4.1 liters
c.)12.3 liters
d.)16.5 liters