How can you distinguish
between living organisms
and nonliving materials
in a water sample?

A living thing or a living cell will most likely take in water from a water solution as the concentration of solutes will be more in the cell. If however, the concentration of solutes is more in water solution, then the living cell will take in the solutes from the water.

As for nonliving things, none such phenomenon will take place. The cell will not have the ability to take in or take out water.

For example, if we place a cell in water which has lower concentration of salts, then water will move inside the cell as the concentration of salts is more inside the cell. Hence, the cell will grow in size due to taking in of water. On the other hand, when a non-living thing is places in water of any kind, none such actions will take place.

they are all solids.

the answer is b because, investigating whether another species of moth also drinks tears might be interesting, but it would not directly explain why this species of moth does.

to explain why this species of moth drinks tears, the scientist could ask: "what nutrients that moths need are in elephant tears? "

the scientist could experiment with moth repellent or oter ways to keep the moths away, but that would not answer his original question.