A candle is placed in front of a concave mirror. The image produced by the mirror is:
Virtual, upright and magnified
Real, inverted and magnified
Virtual, upright and demagnified
Real, inverted and demagnified

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 .