60. The RNA-guided Cas9 nuclease and clustered regularly interspaced short palindromic repeats (CRISPR) enable efficient genome engineering in eukaryotic cells. Which of the following statements about this system is correct? a) originally belong to the microbial adaptive immune system, b) RNA- guided Cas9 nuclease cleaves genetic elements, c) engages DNA repair system, d) a useful tool for specific gene knockdown, e) insertion or deletion usually leads to frameshift mutations. (A) abd (B) abcd (C) bde (D) abce (E) all