Here are the reordered sentences:

6. They volunteered to help children in mountainous areas.
7. How often do you dust the furniture?
8. There is a shop near my house that sells cheap DVDs.
9. I think I have lost my warranty card, and I will pay for any repair.
10. Some word puzzles that people think improve their leisure time are important activities such as reading and doing.

Regarding the underlined parts:
11. (A) should be corrected to "that I" instead of "I that".
12. (A) should be checked for clarity but is acceptable as is at first glance.

Let me know if you need further assistance!