Select the letter of the correct answer. What is this article mainly about?
A. New research has created a type of exercise machine that
helps astronauts maintain their strength in space.
B. New research suggests that astronauts should not
exercise one to two hours in space unless they have
enough to eat first.
C. New research suggests that accelerated space aging is
temporary and that astronauts' muscles recover after a
few months back on Earth.
D. New research suggests that extended space missions may
be a problem because astronauts become weak when they
spend months in space.