How can i tell the difference between a single-replacement reaction and a double replacement reaction?

Single replacement is a reaction in which one element reacts with another compund to form another compound and another element.

example:

A+BC > B+ AC

Double Replacement is a reaction in which the metals present in two compounds change places to form two new compounds 

example:

AB+ CD > AD+ CB (they basically switch partners)

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 .