Parts of speech basically explain the "function" that a word has in a sentence.
This sentence can be divided into 7 parts.
1. Most snakes: that would be the noun since they are the animals that we talk abput in the sentence.
2. Are: That would be the verb that help us describe the noun.
3. Not poisnous: Adjective, I´ts a word that is describing the noun.
4. And: It´s a conjungtion since is joining two ideas together.
5. Rarely: This is an adverb since most adverbs end with "ly" and describe verbs, in this case the verb described is attack.
6. Attack: Verb
7. US: This is a noun since it´s making reference to people.