The answer to the question above is letter "C. enables humans to live forever". Human beings, as we are, we are bound to face death when time comes and that we are not immortal. Other choices are beneficial uses of the genetic engineering.
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Marathon’s consist of running/jogging/sprinting long distances, so first off it would be a good idea to increase stamina, which would be done by jogging everyday, increasing your distance everyday. Stretch your calf muscles upper leg muscles to avoid cramps while running too.
