Why B?

@farnoushsalimian, Since this is a most strongly supported question, it requires us to make an inference that must be true based on the statements in the stimulus. The stimulus tells us that French divers recently discovered a large cave along the coast of the Mediterranean Sea, which is only accessible by an underwater tunnel. The interior is completely filled with seawater and contains numerous large stalagmites. Stalagmites form when drops of water fall repeatedly on a single spot on a cave floor, leaving behind mineral deposits that accumulate over time. This fact tells us that at some point the cave must have been not filled with water, otherwise, it would be impossible for drops of water to "fall" on the floor, resulting in the buildup of mineral deposits. (B) accurately expresses this inference, the water level is higher now than it once was as at some point there was no water on the cave floor, which is what the presence of stalagmites suggests. The rest of the answer choices could be true but they are not must be true since we cannot definitively make any other inferences, we can eliminate everything except for (B). Let me know if this makes sense or if you have any other questions.