根据首字母和汉语提示完成单词。

1. The woman has studied English s______1990.?

2. —Have you heard a______   Lei Feng?

   —Yes, of course.?

3.The plane has a______   in Tianjin already?

4. In fact, since 1989 ______ (工程)Hope has built schools all over China.?

5. How long has Lingling ______ (认识)Sally?

请从消费和生产两个方面简单说明“无形之手”的原理。