D. 5
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C. Experimental
Explanation - The cause and effect of a process would be studied in experimental investigations. However, in descriptive studies, relationship between the factors are not studies rather the detail of the process is studied. In comparative investigation, several factors are compared and its effects are analyzed.
In experimental investigations, experiments are carried out based on the hypothesis and then it is validated accordingly. Validation is a must to prove and disprove the hypothesis. Therefore, experimental data and hypothesis are related.
The two cells of the similar shape, and size have different levels of the ongoing metabolic activity. On one hand, the cell a is metabolically quiet, which means that no energy consuming chemical reactions are taking place in this cell. On the other hand, the cell b is actively consuming the surrounding oxygen. In this scenario, the oxygen will more quickly diffuse into the cell b because the diffusion gradient for oxygen in this cell is steeper than in the cell a.
Hence, the blanks can be filled with 'b and the diffusion gradient is steeper' respectively.
