複選題21. Which of the following descriptions of RNA is (are) CORRECT? (A) The major RNA species in a cell are ribosomal RNA (rRNA) and transfer RNA (tRNA). (B) The poly（Ａ） tail is a characteristic feature of coding RNAs; all noncoding RNAs lack a poly（Ａ） tail. (C) Charged tRNAs are essential for protein synthesis. Although there are more than 40 different tRNA sequences in the cell, they all fold into the same general shape. (D) RNA can fold into various structures, allowing some RNAs to perform enzymatic functions. (E) Ribosomal RNA (rRNA) is the major component of ribosomes.