Rob can't find much time to excercise, but he knows he has to do some activi t to keep his heart healthy. Which of the following is most likely to benefit him, if he remains in his target heart rate zone?
A. Biking to the convenience store (5 minutes)
B. Running o work instead of driving (20 minutes)
C. Swimming after school (10 minutes)
D. Lifting weights while on the phone (30 minutes)