Various investigations have found that the transmission of electrical nerve impulses along neurones becomes slower as age increases. Studies A, B, C and D have reported different outcomes. The average speed of a nerve impulse in people who are 40 years old is $55\,\text{m s}^{-1}$. Which study showed the smallest decrease in transmission speed between the ages of 40 and 60?
- AThe speed reduced by an average of $0.14\,\text{m s}^{-1}$ per year.
- BThe speed reduced by $8.2\,\text{m s}^{-1}$.
- CThe speed reduced by $10\%$.
- DThe speed reduced by $10\,\text{m s}^{-1}$.