In the sentence i want to sleep when im tired the words to sleep are a

In the sentence I want to sleep when I'm tired the words 'to sleep' are an infinitive phrase. Infinite phrases are just verb forms and they can be very easy to pick out because they always have a 'to' in front on them.
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An action or an infinitive phrase

C. It is impossible to sneeze with your eyes open.

Explanation: The correct sentence is indicative in its verb mood because it states a fact. Most sentences in English are in the indicative mood. They simply state a fact or give factual details.