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?

需要性原则是指选题必须着眼于社会和经济发展的（），着眼于体育发展和（）的客观需要。