The subject is understood in 'C'.
The complete sentence, including the subject, would have been " You shake it well before pouring...".
The correct answer is option 'C', where the subject is understood as 'you' in the command 'Shake it well before pouring it into the glass.' This sentence implies the person being addressed without explicitly stating 'you.' Understanding how subjects can be implied is an important aspect of English grammar, particularly in imperative sentences.
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