Meaning: 'Harmful' means causing or capable of causing damage or injury, which is appropriate in the context of toxic waste residues.

Wrong options:

• painful: 'Painful' refers to physical or emotional suffering, which does not apply to the nature of waste residues.
• malicious: 'Malicious' implies intent to harm, which is not relevant to the description of waste residues themselves.
• cruel: 'Cruel' denotes a lack of compassion, which does not accurately describe the residues in a factual context.

Correct answer: up

Explanation: The phrase 'left up to' is a common idiomatic expression indicating that the responsibility or decision is assigned to someone, fitting grammatically and contextually within the sentence structure.

Meaning: 'Up' in this context means that the responsibility is given to these countries, indicating they must handle the situation.

Wrong options:

• on: 'On' does not convey the same meaning of responsibility and would not fit the idiomatic expression used here.
• by: 'By' suggests a method or means, which does not align with the intended meaning of assigning responsibility.
• for: 'For' implies purpose or benefit, which does not accurately reflect the context of responsibility being assigned.

Correct answer: according

Explanation: The word 'according' is used to reference information or rules, fitting grammatically as a preposition that introduces the legal context and aligns with the sentence's structure.

Meaning: 'According' means in agreement with or as stated by, which is appropriate when discussing legal matters.

Wrong options:

• admitting: 'Admitting' implies a confession, which does not fit the context of stating legal facts.
• agreeing: 'Agreeing' suggests a consensus, which is not relevant to the legal framework being referenced.
• stating: 'Stating' is a verb that does not fit grammatically in this context, as it does not introduce the legal reference correctly.

Correct answer: well-established

Explanation: The term 'well-established' describes principles that are recognized and accepted, fitting grammatically as an adjective that modifies 'principles' and contextually aligns with the idea of recognized legal standards.

Meaning: 'Well-established' means firmly established and widely accepted, which is suitable for describing principles in international law.

Wrong options:

• well-educated: 'Well-educated' refers to someone who has received a good education, which does not apply to principles.
• widely-dispersed: 'Widely-dispersed' suggests a physical distribution, which is not relevant to the nature of principles.
• widely-available: 'Widely-available' indicates accessibility, which does not accurately describe the nature of legal principles.