Using a Standard Treadmill Test, I Observe That Today's 30-Year-Olds

Multiple Choice

Using a standard treadmill test, I observe that today's 30-year-olds are more aerobically fit than are today's 60-year-olds. Which of the following is the LEAST plausible explanation of this observation?

A) Fitness is more highly valued in today's society, so the younger cohort exercises more regularly than the older cohort does now or did when they were 30.
B) In U.S. society, jobs and lifestyles become more and more sedentary as adults get older. The observed difference thus reflects a genuine change with age, but not an inevitable one.
C) Inevitable physical changes associated with basic biological aging lie behind the observed difference.
D) It is more difficult to test the aerobic capacity of 60-year-olds, so the findings are probably misleading.