Final answer:
The subject in the sentence 'The sun set as the moon began to rise' is sun.
Step-by-step explanation:
In the given sentence, the subject is sun. The subject of a sentence is the noun or pronoun that performs the main action of the verb. In this case, the sun is the subject because it is the one that sets, indicating the action being performed. The moon, on the other hand, is not the subject but rather the object of the action, as it begins to rise.
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