Here are the corrections for each of the sentences:

1. **The result of that test must be informed before August.**
(Correct answer: B)

2. **She has disappeared three days ago, and they are still looking for her now.**
(Correct answer: B)

3. **The children were so amusing at the cartoon that they couldn't help laughing.**
(Correct answer: B)

4. **My uncle Hung, who has come back from England, as well as his four children, are warmly welcome.**
(Correct answer: B)

5. **George hasn't completed the assignment yet, and Maria hasn't too.**
(Correct answer: B)

6. **My parents let me go out without saving anything.**
(Correct answer: C)

7. **They stopped to work because of the hot weather.**
(Correct answer: B)

8. **They worked very hardly in spite of their old age.**
(Correct answer: C)

9. **They have lived next door to the US for years; therefore, we hardly ever see them.**
(Correct answer: C)

Feel free to let me know if you need any more assistance!