Meaning: 'Through' means by way of or by means of, indicating the method of how heavy metals enter lakes and rivers.

Wrong options:

• within: 'Within' suggests being inside something, which does not convey the idea of movement into lakes and rivers.
• inside: 'Inside' implies being contained within something, which does not fit the context of carrying metals into water bodies.
• across: 'Across' indicates movement from one side to another but does not imply the method of carrying, making it less suitable in this context.

Correct answer: being

Explanation: The word 'being' is the correct form here as it indicates the ongoing action of metals being released into the environment. It fits grammatically as a gerund that complements the structure of the sentence.

Meaning: 'Being' refers to the state of existing or occurring, indicating that the release of metals is an ongoing process.

Wrong options:

• were: 'Were' is a past tense verb that does not fit the present context of the sentence, which discusses current actions.
• to be: 'To be' is an infinitive form that does not fit grammatically in this context where an ongoing action is described.
• are: 'Are' is a present tense verb but does not correctly convey the continuous aspect of the release of metals.

Correct answer: major

Explanation: The word 'major' is appropriate as it signifies something of great importance or significance, which aligns with the context of sources of heavy metal poisoning. It fits grammatically as an adjective describing the sources.

Meaning: 'Major' means significant or important, indicating that these sources are key contributors to heavy metal poisoning in humans.

Wrong options:

• great: 'Great' can imply size or intensity but does not specifically convey the importance of the sources as effectively as 'major.'
• large: 'Large' refers to size rather than significance, making it less suitable in this context.
• considerable: 'Considerable' implies a large amount but lacks the direct implication of importance that 'major' provides.

Correct answer: despite

Explanation: The word 'despite' introduces a contrast, indicating that even with the serious health risks, soil contamination remains a problem. It fits grammatically as a preposition that sets up this contrast effectively.

Meaning: 'Despite' means in spite of, indicating that the problem persists regardless of the serious health risks involved.

Wrong options:

• although: 'Although' is a conjunction that introduces a dependent clause, which does not fit the structure of the sentence as effectively as 'despite.'
• despite: This option is actually the correct answer, so it should not be included in the wrong options.
• nonetheless: 'Nonetheless' is an adverb that indicates a concession but does not fit as smoothly into the sentence structure as 'despite.'
• regardless: 'Regardless' implies without regard to something, but it does not convey the same contrastive meaning as 'despite' in this context.