A Randomized Experiment Was Done by Randomly Assigning Each Participant

A randomized experiment was done by randomly assigning each participant either to walk for half an hour three times a week or to sit quietly reading a book for half an hour three times a week. At the end of a year the change in participants' blood pressure over the year was measured, and the change was compared for the two groups.
-If a statistically significant difference in blood pressure change at the end of a year for the two activities was found, then

A) it cannot be concluded that the difference in activity caused a difference in the change in blood pressure because in the course of a year there are lots of possible confounding variables.
B) whether or not the difference was caused by the difference in activity depends on what else the participants did during the year.
C) it cannot be concluded that the difference in activity caused a difference in the change in blood pressure because it might be the opposite, that people with high blood pressure were more likely to read a book than to walk.
D) it can be concluded that the difference in activity caused a difference in the change in blood pressure.