The addition of 6 g of salt to a sample of pure water causes its freezing point to be reduced from 0°C to -1.5°C. After an additional amount of salt is dissolved in the solution, its freezing point is -4.5°C. What is the total amount of salt dissolved in the sample of water?
A. 9 g
B. 12 g
C. 15 g
D. 18 g