What is a mixture of 2 or more substances in which the particles cannot be seen?

An experiment is conducted to see if cats preferred skim milk or 2% milk. a cup of skim milkwas put out for 5 kittens and then measured how much the kittens drank over the course of aday. following a cup of 2% milk was purout for the skittens and then masured how much thekittens drank over the course of a day. the same kittens were used and the milk was served atthe same temperature. it was discovered that the cats liked the 2% milk more than the skimmilk. what is the dependent variable in this experiment?