Task: Question Answering
Input (English): Question: Ultraviolet radiation changes the structure of DNA by: 
 Options:  <1> Rupture of phosphodiester bonds. <2> Deamination of nitrogenous bases. <3> Depurinization <4> Formation of cyclobutyl rings of pyrimidines. <5> Hydroxylation of nitrogenous bases.
Output (English): 4