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YOUR ANSWER: Three is the subject and plus is the verb.
Three is the subject, all right enough, but plus is not a verb.
Now, you may think the word plus should be a verb because ordinarily when you see a plus sign you regard it as an instruction to do something; that is, you think it says "add", so you add. But this adding is something that you do, and not something that the subject of the sentence does. If the word plus were a verb, then the expression two plus one would of necessity be a sentence in itself, and it's not. Two doesn't plus one or do anything to one. Two and one are just sitting there with the word plus in between them defining their relationship to each other. This relationship can be consumated by adding them together---but it's up to you to do it.
In grammar we learned that words which define relationships are called prepositions. Some familiar prepositions are the words on, at, with, for, to and so on. The word plus is such a preposition.
Now return to page 7 and choose the correct answer.