N. C. and D.-D. H. Yang discovered a reaction in which this phenomenon causes C-H functionalization. Reactions involving this phenomenon are often promoted by adding a catalytic amount of benzophenone. The first organic reaction known to involve this phenomenon is the rearrangement of santonin. (-5[1])This phenomenon cleaves a carbonyl in both types of Norrish reactions. (-5[1])In one biosynthetic pathway, this phenomenon causes a ring opening in (*) 7-dehydrocholesterol. This phenomenon causes the formation of a cyclobutane ring connecting two adjacent pyrimidines, creating thymine dimers (10[1])and consequently causing DNA damage. (10[2])This phenomenon causes the formation of molecular oxygen and oxygen radicals as part of an atmospheric cycle that absorbs this phenomenon. (-5[1])For 10 points, name this component of sunlight (-5[1])used in the biosynthesis of vitamin D. ■END■ (10[3]0[1])