How many people were in each group

A．Most of us don't get enough exercise.
B．We should exercise 15 minutes per day.
C．A high pulse rate is more healthy.
D．Regular exercise can strengthen our heart.

A．at the beginning and at the end of the 7-week period
B．after each exercise period
C．only at the beginning of the 7-week period
D．only at the end of the 7-week period