Zinc finger nucleases could be used to solve one major disadvantage of using retrovirus vectors in gene therapy treatments.Which of the following disadvantages could they solve?

A) The low levels of transgene expression.
B) The stimulation of the immune system by the virus vector.
C) The random integration into the host genome.
D) The ability to only insert small transgenes in the vector.
E) The toxicity of the viral proteins to the transduced cell.