A lift is held up by two steel cables, each with a length of $20\ \text{m}$. Each cable is made up of $100$ parallel steel wires, and each wire has a cross-sectional area of $3.2 \times 10^{-6}\ \text{m}^2$. The Young modulus of steel is $2.1 \times 10^{11}\ \text{N m}^{-2}$. How far does the lift move downwards when a man with mass $70\ \text{kg}$ gets into it?
- A$0.010\ \text{mm}$
- B$0.020\ \text{mm}$
- C$0.10\ \text{mm}$
- D$0.20\ \text{mm}$