Why might using retroviral vectors for gene therapy increase the patient's risk of developing cancer?

A)Retroviral vectors might introduce proteins from the virus.
B)Retroviral vectors might not express the genes that were introduced into a patient's cells.
C)Retroviral vectors might not integrate their recombinant DNA into the patient's genome.
D)Retroviral vectors might integrate recombinant DNA into the genome in ways that misregulate the expression of genes at or near the site of integration.
E)Retroviral vectors might cause a strong immune response.