The diagram illustrates a system designed to prevent trains from going above a safe speed. Trains approaching from the left move with a speed of $50\,\text{m s}^{-1}$. At marker 1, the driver has to use the brakes so that the train slows down at a constant rate and reaches marker 2 with a speed of no greater than $10\,\text{m s}^{-1}$. The train has a detector which records the times at which it passes each marker and activates an emergency brake if the interval between passing marker 1 and marker 2 is under $20\,\text{s}$. At what distance from marker 2 should marker 1 be positioned?
- A$200\,\text{m}$
- B$400\,\text{m}$
- C$500\,\text{m}$
- D$600\,\text{m}$