Which of the following two scenarios has two independent samples and why?

a) Determine the difference in stopping distance when going 60mph on dry and wet pavement. Use 20 cars ( For 10 of the cars measure stopping distance on dry pavement and for the other 10 measure the stopping distance on wet pavement)

b) Determine the difference in stopping distance when going 60mph on dry and wet pavement. Use 10 cars ( For each of the 10 cars measure stopping distance on dry pavement and then measure the stopping distance on dry pavement).