A study of the impact of caffeine consumption on reaction time used 24 subjects and divided them into 12 pairs based on the average hours of sleep they got. One randomly assigned member of each pair drank two cups of caffeinated coffee, and the other drank two cups of decaf. Each subject's performance on a standard reaction time test was recorded. The difference for each pair is calculated (caffeinated - decaf). A 95% confidence interval for the mean difference in reaction times is (-1.2, 0.75).

What is a valid conclusion they can make?

a. Because the confidence interval includes more negative numbers than positive, we have convincing evidence that caffeine decreases reaction time.

b. Because the confidence interval includes 0, we don't have convincing evidence that there is a difference in reaction time.

c. Because the center of the interval is -0.225, we have convincing evidence that decreases reaction time.

d. Because the confidence interval includes 0, we have convincing evidence that the true mean difference is 0.