Answer:
They should be blocked by body fat composition so the subjects are as similar as possible.
Step-by-step explanation:
It's D .
Blocking here means how should the subjects be arranged in terms of body fat or weight.
Option A is wrong because alphabetically controlling body fat would not get desired results.
Option B is wrong because different age groups would yield different results.
Option C is wrong also because weight loss is not dependent on taller people.
Option D is correct because controlling body fat would yield best possible results in ascending or descending order
Option E is incorrect because arranging data in ascending or descending order would simplify the researches experiments.
