Final answer:
In an environment without gravity, concerns include the loss of bone mass, muscle atrophy, cardiovascular health issues, and balance problems. All these factors need to be addressed when astronauts spend extended periods in space, and they use special equipment to exercise and maintain fitness. Therefore correct option is D
Step-by-step explanation:
When considering an environment without gravity, such as what astronauts experience in outer space, you must worry about d. All of the Above: a. Bones and muscle loss, b. Cardiovascular health/dizziness, and c. Balance. Extended stays in microgravity, as on the International Space Station, result in muscle atrophy and a loss of bone mass due to the lack of gravitational force that would typically stress and maintain bone and muscle integrity on Earth. Moreover, the cardiovascular system adapts in space since there is no pressure gradient caused by gravity, which affects blood pressure and the return of blood to the heart.
To mitigate these effects, astronauts use specially designed equipment to exercise and simulate weight-bearing activities that help maintain body fitness and counteract the deleterious effects of living in microgravity.