Larry Spends All His $800 Monthly Income on Pizza and Milk

Larry spends all his $800 monthly income on pizza and milk. The price of pizza is $4 a slice, and the price of milk is $2 per litre. The relative price of milk is

A) 2 slices of pizza per litre.
B) 0.5 slices of pizza per litre.
C) 0.5 litres per slice of pizza.
D) $2 per litre.