IV. Each of the following sentences has one mistake. Identify and correct the mistakes. 

16. While we were on holiday, we spend most of our time sightseeing. 

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17. Paul listened to his iPod, so he didn’t hear the doorbell ringing. 

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18. He is a most intelligent student in our class 

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19. Your new car is more cheaper than John’s. 

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20. He left college when he is 17. 

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