Which is the best time to apply sunscreen? before going to bed just after going out in the sun 1 hour after going out in the sun 30 minutes before going out in the day

30 minutes before going out in the day

Copper, silver, aluminum, cobalt, chromium, mercury 

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 .