Sentence I talks about a lost contact and sentence II talks about the arrival at a place. It can be noted that sentence II is a condition or reason that may have led to the situation in sentence I. So, the connector/s must indicate the relation between the two sentences.
Accordingly, 'as' (used to indicate that something happens during the time when something else is taking place), 'because' (for the reason that; since) and 'since' (from a time in the past until the time under consideration, typically the present) are the connectors that match the context of the given sentences and can thus be used.
Nor- used to introduce a further negative statement
Hence, the correct answer is option (a).
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