Hi Sarthak

The first sentence of the passage states: "If 10% or more of a planet's total mass is water, there can be no freshwater environment on that planet as bodies of freshwater are produced by accumulation of natural precipitation on land."

We should look at this in three parts - the part before the comma, the conclusion after the comma (please note this may or may not be the final conclusion of the passage) and the premise on which that conclusion is based.

The portion before the comma is very simple, it provides a condition, say, X: "If 10% or more of a planet's total mass is water"

The first portion after the comma is the conclusion, which is also the effect, say, Y, of the above condition: "there can be no freshwater environment on that planet"

The last portion after the comma provides the premise, or reason (say, Z), for the conclusion or effect to be true: "bodies of freshwater are produced by accumulation of natural precipitation on land"

So the structure is: If X, then Y because Z.

I hope this helps.
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I chose c, I forgot phosphorus needs to be in absorbable form. Very tricky.