In preparation for a blizzard, a city dispatches a crew to spread salt on the street surface of the bridges around town. the salt is spread on the streets because it will likely change the freezing point of water from to

It's a

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 .

The answer is the third choice