An active student injured his knee playing basketball and has been advised by his doctor to avoid impact activities that cause heavy force on his bones. what is the most effective activity he can do to maintain his stamina without further injury?
a. basketball
b. jogging
c. swimming
d. jumping rope

Jogging.  I found this answer by process of elimination.  Swimming takes a lot of force, like kicking which gives heavy force on bones, jumping rope, (believe it or not) can also affect his bones since he is "pounding" on the ground.  Basketball is much like the jumping rope.  But jogging is meant to be a light run, and if you jog the right way, (lightly on your feet) you can stay active and avoid heavy force on bones.

"air pollution" is the correct answer i believe