Which of the following is the CORRECT definition of O-notation?

1. AO(g(n)) = {f(n): there exist positive constants c and n₀ such that 0 ≤ f(n) ≤ c·g(n) for all n ≥ n₀}
2. BO(g(n)) = {f(n): there exist positive constants c and n₀ such that 0 ≥ f(n) ≥ c·g(n) for all n ≥ n₀}
3. CO(g(n)) = {f(n): there exist positive constants c₁, c₂ and n₀ such that 0 ≤ c₁·g(n) ≤ f(n) ≤ c₂·g(n) for all n ≥ n₀}
4. DO(g(n)) = {f(n): there exist positive constants c and n₀ such that 0 ≤ c·g(n) ≤ f(n) for all n ≥ n₀}