Which sentences are opening (O) / closing (C) remarks?

1. I must go now. (C)
2. It was nice to hear from you. (O)
3. Well, I think that's about it. (C)
4. How are you? (O)
5. Thanks for your email. (O)
6. I haven't heard from you for ages. (O)
7. Write soon. (C)
8. Well, that's all for now. (C)