After verbs of perception, we use the base form (infinitive without "to") or the active participle (the -ing form), so choice D is correct. We cannot use a conjugated verb (went, goes, was) without a subordinating conjunction such as "as" or "while", so choices A, B, and C are wrong. Note that "sun" is grammatically the *object* of "watch", not the subject of "go".

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