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2 answers:
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Answer: había bebido
Cuando llegué, Marcela ya se había bebido cuatro vasos de leche.
When I arrived, Marcela had already drunk four glasses of milk.
Explanation: the Spanish plusquamperfect expresses the idea that something occurred before another action in the past. The action to arrive, it happened after the drinking action was over. In English this tense is named past perfect.