I believe the answer is C (recycle them, but not through the regular trash disposal)
I hope this helps :)
A sole carbon particle: The degradation of particulate organic matter in the ocean is a central process in the global carbon cycle, the mode and tempo of which is determined by the bacterial communities that assemble on particle surfaces.
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.