複選題2. Which one of the following statements about mRNA stability is true? (A) Degradation always proceeds in the 5' to 3' direction. (B) 5' poly（Ａ） tail protects RNA from degradation. (C) Secondary structure in mRNA (hairpins, for example) slows the rate of degradation. (D) In general, bacterial mRNAs have longer half-lives than do eukaryotic mRNAs. (E) Rates of mRNA degradation are always at least 10-fold slower than rates of mRNA synthesis.