The surfaces of leaves contain guard cells that open and close the stomata. What is the primary function of the guard cells?

They control the loss of heat produced by the plant.

They control the loss of water through transpiration.

They control the loss of carbon dioxide used during respiration.

They control the loss of sugars produced during photosynthesis

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 .