Ответ:
1 The Earth climate becomes hotter and hotter.
2 Our trip became harder as it began to snow.
3 The plane tickets are the most expensive.
4 I agree that Kate is more serious than (the) others.
5 As the time went on we became more nervous because he didn’t come.
6 The pollution of environment is becoming worse and the problem to solve (it is) the most difficult.
7 Dolphins are the cleverest/the most clever mammals.
8 He is my best friend.
Объяснение: