15. Mount Snowdon is the lowest mountain. Mount Fuji is higher than Mount Snowdon. Mount Everest is the highest mountain. The river Danube is the shortest river. The river Amazon is longer than the river Danube. The river Nile is the longest river.
16. John's car cost £15,000 Jane's car cost £25,000 Bill's car cost £55,000 John's car is the least expensive car. It's the cheapest car. John's car was less expensive than Jane's car. It's cheaper than Jane's car. Bill's car is the most expensive car.
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18. sad sadder the saddest Sad Sadder The saddest Happy Happier The happiest
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21. 1. In the 1 st picture it is hotter than in the 2 nd picture. 2. In the 2 nd picture it is hotter than in the 1 st picture.