Following are the steps in one type of gene therapy. (i) Inject engineered cells into patients bone marrow. (ii) Viral DNA earring the normal allele inserts into chromosome. (iii) Let retrovirus infect bone marrow cells that have removed from patient and cultured. (iv) Insert RNA version of normal allele into retrovirus. The correct sequence is:

• A. (i), (ii), (iii) and (iv)
• B. (iv), (iii), (ii) and (i)
• C. (i), (ii), (iv) and (iii)
• D. (iv), (iii), (i) and (ii)