Which expression represents a reaction rate?
a. time/mass
c. energy/time
b. number/time
d. time/energy

C. Energy/Time is the expression that represents a reaction rate.

Reaction rate is the speed at which a chemical reaction happens. Because it involves speed, time must be present. Chemical reaction involves energy. So a reaction rate must be energy over time. 

Answer is to avoid experimental error

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 .