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Answer: C. Day in, day out, on the road.
Explanation:
The sentence that expresses the idea of a routine and repetitiveness is sentence C. As explained in the question itself, in 1915, people lived mundane lives and for them, all the days were practically the same - they would wake up, have breakfast, go to work, return home, sleep, and wake up only to do the same as the previous day. Kafka wanted to include this aspect of society in his Metamorphosis as well, in order to make it as truthful as possible.
The United States and Russia maintain diplomatic and trade relations.
