Scenario Patients with Organ Failure or Severe Diseases May Need Organ

Scenario
Patients with organ failure or severe diseases may need organ transplants. However, there is often a long waitlist for certain organs, and patients may not be able to acquire a human organ transplant in time. To address this issue, pigs have been studied as a potential source of organs to use for transplants because pig organs are similar in size to human organs and are easy to obtain. However, a major health concern that needs to be addressed before pig organ transplantation becomes viable is the prevention of bacterial or viral transmission from pig organs to the human patient.
One pig virus that is particularly difficult to prevent from being transmitted is porcine endogenous retrovirus (PERV) . At some point in time, PERV infected pig eggs or sperm and thus incorporated its own genome into the pig genome, and all species of pigs carry the PERV genome to some extent. Therefore, if a pig kidney is used for a human transplant, all of the pig kidney cells will have the DNA for PERV, and these could possibly infect the human host.
To prevent transmission of PERV, researchers have recently used CRISPR-Cas9 gene editing to introduce mutations in the pol gene. The pol gene codes for the enzyme reverse transcriptase that functions to produce PERV from the pig genome, thus preventing PERV from being produced. As of late 2017, 37 CRISPR-edited piglets were born and 15 were still alive. However, their organs have yet to be transplanted into humans.
-What would have been the function of Cas9 when using CRISPR-Cas9 to edit piglets' genomes?

A) to repair mutations in the pol gene
B) to remove the pol gene from the pig genome
C) to insert nucleotides into the pol gene
D) to cut the pol gene