句型转换（每空一词） 。
1. They heard a message from a spacecraft. (用just改为现在完成时态形式)
They just _______ a _______ message from a spacecraft.
2. Have you got a letter from Lucy?(改为同义句)
Have you _______ _______Lucy?
3. To understand the problem is very difficult.(用it作为形式主语)
_______ _______very difficult _______understand the problem.
4. I came here two days ago.(改为同义句)
I _______ _______ here _______ two days.?
5. Her mother has been to Hong Kong several times.(就画线部分提问)
_______ _______ _______ has her mother been to Hong Kong?
