Discussion
Phytoplanktons have extremely small diameter because :
(i) it provides relatively high surface area to volume ratio.
(ii) it attributes to frictional resistance to sinking.
(iii) it helps in efficient uptake of nutrients.
(iv) it helps to navigate efficiently in sea water.
Answer: A
Statements i, ii and iii are true, so the answer is Only (i), (ii) and (iii) are true.
Being very small gives phytoplankton a high surface area to volume ratio (i). That large surface relative to their mass increases frictional drag and so slows sinking, helping them stay in the sunlit layer (ii). The large surface also allows efficient uptake of dissolved nutrients by diffusion (iii).
Statement iv is false: phytoplankton are drifters that cannot navigate against currents, and small size does not give them navigational ability. That rules out 'all are true', and the options leaving out i or ii are incomplete.
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