This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Biochemistry
All are true about CRISPR cas 9 Except 
Select and explain. 
A. gRNA ensures that the Cas9 enzyme cuts at the right point in the genome, B. NHEJ repair & HDR pathway, C. C9 enzyme is used in CRISPR gene editing, D. All of these
D: All of these
Answer: D: All of these 
Explanation: Answer-D. All of theseCRISPR-Cas9 was adapted from a naturally occurring genome editing system that can generate double standard Breaks (DSBs).Cas9 is the enzyme that is used most often, other enzymes (for example Cpf1) can also be used- these enzymes can can repair by homologus repair system or by non-homologous end joining in the absence of DNA template.Two general repair pathways:The efficient but error-prone non-homologous end joining (NHEJ) pathwayThe less efficient but high-fidelity homology directed repair (HDR) pathway