Nancy wants to find out how the steepness of a hill's slope affects the amount of soil that erodes from it. she has soil, a long pan, and a pile of books with which to build a model hillside. she has a watering can to model rainfall. in her experiment, which of the following factors should nancy make sure that she does not change?

The independent variable is the type of music.the dependent variable is the pulse rate.the intervening variable is the age of the subject.if the mother does not like rock, her pulse is likely to not increase.the same might be true for the sister.the friend may like rap and gets excited.to establish cause/effect relationship, all of the subjects must be consistent.hope i .