31. ________ in the atmosphere is the temperature falling below freezing. A. Frost is produced B. Frost produces C. What produces frost D. What is frost produced 32. He said that the plane had already left and that I________ an hour earlier. A. Must have arrived B. had to arrive C. should have arrived D. was supposed to arrive

31. ________ in the atmosphere is the temperature falling below freezing.

A. Frost is produced  B. Frost produces  C. What produces frost  D. What is frost produced.

Giải thích: - Frost: là hiện tượng trong thiên nhiên, -> sử dụng CĐ không dùng BĐ.

                 - "What" đứng đầu câu để nhấn mạnh vào chủ ngữ sau đó của câu.

32. He said that the plane had already left and that I________ an hour earlier.

A. Must have arrived  B. had to arrive  C. should have arrived  D. was supposed to arrive

Giải thích: Should have done: một việc lẽ ra nên làm trong quá khứ nhưng lại không làm.

Đảm bảo chính xác :3

Xin hay nhất ạ :3