Meaning: 'Volume' refers to the amount of space that a substance (in this case, fresh water) occupies, which is relevant to understanding the salinity of the sea.

Wrong options:

• pressure: Pressure refers to the force exerted by a substance, which does not relate to the quantity of fresh water.
• depth: Depth indicates how deep something is, not the amount of fresh water received, making it contextually irrelevant.
• rate: Rate refers to the speed of something, which does not apply to the quantity of fresh water affecting salinity.

Correct answer: alone

Explanation: The word 'alone' is appropriate here as it emphasizes that the Volga River is the sole contributor to the majority of the incoming water, fitting both grammatically and contextually within the sentence.

Meaning: 'Alone' means without others or solely, highlighting that the Volga is the primary source of water for the Caspian Sea.

Wrong options:

• slightly: Slightly is an adverb that indicates a small degree, which does not fit the context of contribution as it does not convey the idea of being the only contributor.
• lonely: Lonely describes a state of being alone in a sad way, which is not relevant to the context of water contribution.
• slight: Slight is an adjective that means small in degree, which does not convey the intended meaning of being the only contributor.

Correct answer: uniform

Explanation: The word 'uniform' is suitable as it describes the consistency of salinity levels, indicating that they are not the same throughout the Caspian Sea. This aligns with the context of discussing variations in salinity.

Meaning: 'Uniform' means consistent or the same in all parts, which is relevant to the discussion of salinity variations in the sea.

Wrong options:

• impartial: Impartial means unbiased or neutral, which does not apply to the concept of salinity levels in this context.
• confident: Confident describes a state of certainty or assurance, which is unrelated to the salinity levels being discussed.
• shifting: Shifting implies change or movement, which does not accurately describe the nature of salinity being consistent or varied.

Correct answer: affect

Explanation: The word 'affect' is the correct choice as it indicates that the differences in salinity have an impact on marine life and ecological balance, fitting grammatically and contextually within the sentence.

Meaning: 'Affect' means to have an influence on something, which is relevant to how salinity impacts marine ecosystems.

Wrong options:

• defect: Defect refers to a flaw or imperfection, which does not relate to the influence of salinity on marine life.
• deflect: Deflect means to cause something to change direction, which is not applicable in the context of salinity's impact.
• effect: Effect is a noun meaning a result, which does not fit grammatically as the sentence requires a verb.