Here are the correct answers for the sentences provided:

6. **B** (relatives who I had never met before)
7. **C** (people who live in the same building)
8. **A** (Human beings are said to be responsible)
9. **D** (a colorful poster which consists of a group of young people who were dancing)
10. **B** (I would like to write about several problems which I have faced since I came to the United States.)
11. **A** (Laws have been introduced to prohibit the killing of rare animals)
12. **A** (It is almost impossible to find two people whose opinions are the same.)
13. **B** (Due to the shortage of funds, it was impossible to implement the projects which we were very much)
14. **A** (We are changing the environment by building cities and villages where forests once stood.)
15. **C** (There is a small wooden screen that separates the bed from the rest of the room.)

Keep in mind the context for correct answer choices is dependent on grammatical accuracy.