Marcus is on a train that travels 20 ms and takes 5 seconds to slow to 10 ms. How would Marcus correctly calculate the acceleration?
A
a = (0−20)5 = -4 m/s2

B
a = (10−20)5 = -2 m/s2

C
a = (20−10)5 = 2 m/s2

D
a = (20−0)5 = 4 m/s2