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Hi! Are these all ok? Thanks, Fulvio

1. The reason is that it is summer in Australia in December.

2. The reason they got married is that they love each other.

3. The reason he is so happy is that he won the lottery.

4. The reason climbing Mt Everest is dangerous is that it is the highest mountain in the world.

5. The reason the T-shirt is cheap is that it is made in China.

6. The reason is that they eat a lot of fast food.

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Yes. Of course, (1) and (6) need extra context to provide the answer to "reason for what?".