Answer: Option B
Step-by-step explanation:
Usually in scientific experiments, at least one parameter is checked; here, the growth rate of wheat in two different conditions is being evaluated.
Hence, growth rate of wheat alone in pans A, B and C becomes the standard or referential control against which the experimental control (Wheat + clover) in pans D, E and F is evaluated, since both treatments are exposed to the same external conditions.
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