Dưới đây là câu trả lời cho từng câu hỏi:

17. "A friend of my cannot connect her phone."
- **Đáp án đúng: C. my** (Câu này nên là "A friend of mine cannot connect her phone.")

18. "What __________ when his mother came?"
- **Đáp án đúng: B. was you doing** (Câu hỏi nên là "What were you doing when his mother came?")

19. "Air ________________, together with littering, is causing many problems in our cities today."
- **Đáp án đúng: B. pollution** (Câu này nên được viết là "Air pollution, together with littering, is causing many problems in our cities today.")

20. "We cleaned up everything at the campsite ______________ we left."
- **Đáp án đúng: B. before** (Câu này nên là "We cleaned up everything at the campsite before we left.")

Tóm lại:
17. C
18. B
19. B
20. B