Oceans did not exist on earth 4 billion years ago because the surface of earth was too for the presence of liquid water.
a. rocky
b. hot
c. cold
d. old

The surface of the earth was too hot for liquid water

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 .