challenge: What is the primary function of the H/ACA class of snoRNAs, to which U23 belongs?
  
  A. To catalyze the splicing of pre-mRNA
  B. To guide the modification of adenines to inosines in RNA
  C. To regulate the transcription of protein-coding genes
  D. To form direct base pairing interactions with substrate RNAs to guide the modification of uridines to pseudouridines
  Answer: 