The presence of which of the following substances can allow a chain reaction to happen?
a) a reactive product
b) a reactive intermediate
c) a beta particle
d) none of the above

I'm not sure but i will try to find out

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 .