Several steps in the manufacture of human insulin are shown below. 1. Genetically modified $E.\ coli$ bacteria are cultured in large fermenters. 2. The gene for human insulin is placed into the DNA of a bacterium named $E.\ coli$. 3. The gene for human insulin is taken from human pancreas cells. 4. Human insulin is then removed and purified. What is the correct order of these stages?
- A$3 \rightarrow 1 \rightarrow 2 \rightarrow 4$
- B$4 \rightarrow 3 \rightarrow 2 \rightarrow 1$
- C$3 \rightarrow 2 \rightarrow 4 \rightarrow 1$
- D$3 \rightarrow 2 \rightarrow 1 \rightarrow 4$