<P> Sunblock and sunscreen are different important skin - care products though both offer full protection from the sun . </P> <P> Sunblock--Sunblock is opaque and stronger than sunscreen, since it is able to block most of the UVA / UVB rays and radiation from the sun, and does not need to be reapplied several times in a day . Titanium dioxide and zinc oxide are two of the important ingredients in sunblock . </P> <P> Sunscreen--Sunscreen is more transparent once applied to the skin and also has the ability to protect against UVA / UVB rays, although the sunscreen's ingredients have the ability to break down at a faster rate once exposed to sunlight, and some of the radiation is able to penetrate to the skin . In order for sunscreen to be more effective it is necessary to consistently reapply and use one with a higher sun protection factor . </P> <P> Vitamin A, also known as retinoids, benefits the skin by normalizing keratinization, downregulating sebum production which contributes to acne, and reversing and treating photodamage, striae, and cellulite . </P>

Which of the following constitutes a function of the skin