In the form of gene therapy used successfully for severe combined immunodeficiency syndrome ...

In the form of gene therapy used successfully for severe combined immunodeficiency syndrome (SCID)-X1, the genetic engineering of human cells is done
A) by injecting engineered viruses into the patient's bloodstream.
B) by injecting engineered viruses into the patient's bone marrow.
C) by treating a relative's cultured bone marrow cells with genetically engineered viruses and then injecting these cells into the patient's bone marrow.
D) by isolating the patient's bone marrow cells, infecting them with genetically engineered viruses, and injecting them back into the patient's bone marrow.