A bacterial cell usually contains a single loop of DNA that is located in the cytoplasm of the cell and is not surrounded by a nuclear membrane. Some bacteria have additional loops of DNA called plasmids. Plasmids can be used for genetic engineering because DNA from other organisms can be spliced into a plasmid loop. For example, scientists in the late 20th century discovered a way to insert the portion of human DNA that codes for the protein insulin into plasmid E. coli bacteria. Which statement best explains why these genetically modified bacteria have been useful to society?
A. Human diabetics are now more resistant to bacteria B. The bacteria are resistant to human diabetics C. The bacteria can now make their own insulin D. the bacteria are used to produce cheap insulin.