複選題35. Regarding to RNA processing which statement is (are) CORRECT? (A) Transcription and processing of pre-rRNA occur in the nucleolus by RNA polymerase I. (B) Pre-tRNAs are synthesized by RNA polymerase III in the nucleoplasm and undergo extensive modification in the nucleus. (C) Nuclear bodies are functionally specialized regions in the nucleus where interacting proteins form self- organized structures, responsible for assembly of ribonucleoprotein (RNP) complexes. (D) mRNAs never occur as free RNA molecules in the cell, but are always associated with proteins as RNP complexes. (E) Different mRNAs may be expressed from the same gene in different cell types or at different developmental stages because of alternative splicing of primary transcripts, the use of alternative promoters, and cleavage at different poly（Ａ） sites.