Which of the following responses has been shown in ground-based spaceflight simulation studles?
a. Smooth muscle hypertrophy of cerebral arteries and diminished ability to vasoconstrict
b. Smooth muscle hypertrophy of cerebral arteries and enhanced ability to vasoconstrict
c. Smooth muscle atrophy of cerebral arteries and diminished ability to vasoconstrict
d. Smooth muscle atrophy and enhanced ability to vasoconstrict