The two statements that most likely come from credible sources are:
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There are four classes of jellyfish, and they are known as Scyphozoa, Cubozoa, Hydrozoa, and Staurozoa.
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Jellyfish can sting humans, and their stings are some of the most painful a person can experience.
These statements contain factual information based on scientific classification and the known effects of jellyfish stings, which are commonly discussed in credible scientific literature and resources.