Cross-training involves
a. playing a variety of sports for exercise
b. performing different types of cardiovascular exercise
c. performing a variety of resistance training exercises
d. all of the above

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during longer space exploration trips, crewmembers may develop vision impairment risks and short-term cardiac atrophy or a decrease in the size, strength, weight, and activity of the heart during space flight because they can experience microgravity. in addition, the long-term medical condition is loss of bone mineral density and radiation exposure.