Answer:
B) the slow rate at which diffusion occurs over large distances.
Explanation:
The complexity of the respiratory system is direct proportional with the size of the organism. As an organism size increases, diffusion begins to take place over a larger distance and the ratio of surface area to volume is seen to decrease. In unicellular organisms, diffusion across the cell membrane is adequate for distributing oxygen to the cell unlike in multicellular organisms.
Diffusion is known to be a slow, gradual and passive transport activity. It is important that for diffusion to be a pratical way of supplying oxygen to the cell, the amount of oxygen intake must be the same as the amount of diffusion across the membrane. This implies that, if the cell happens to be very large or thick, diffusion would not serve as the best means to distribute oxygen swiftly and in the right quantitiy to the inner region of the cell.
We can say that reliance on diffusion as a tool of supplying or distributing oxygen and extracting carbon dioxide is actually attainable only for small size organisms or the ones that possess a highly-flattened bodies, e.g. flatworms (Platyhelminthes).
Thus, this accounts for why Larger organisms had to develop specialized respiratory tissues, such as gills, lungs, and respiratory passages in conjunction with a complex circulatory systems, to disburse oxygen all through their entire body and to compensate for the slow rate at which diffusion occurs over large distances.
