Why is the post-transcriptional poly-A tail an important addition to a mRNA molecule?

• A. The 3' tail signals the end of transcription by the RNA polymerase.
• B. The poly-A tail is a part of the protein translated from the mRNA.
• C. The poly-A tail facilitates the splicing of introns.
• D. The tail adds stability to the transcript, controlling the time of degradation.