If you improve the of the muscles to extract oxygen from the bloodstream
to produce energy, you improve your cardiovascular endurance.
a) Leisurely
b) Buoyancy
c) Capacity
d) Vessel

I think its C. capacity.

Explanation:

when in a crucial situation, after calling 911, the correct posture for delivering effective compression is the proper hand placement.

first, place two fingers at the sternum, then put the heel of your other hand next to the fingers and place hand on top of the other. lock your elbows by using your body’s weight and compress the victim’s chest with a depth of 2 inches.

moreover, do 15 compression and 2 breaths, this process must be done four times until there is a pulse or a arrive.