GPWS（近地警告系统）在无线电高度表（）以下能够产生音响和目视警告？
A、2000英尺
B、2450英尺
C、2500英尺
参考答案：B
Think and write. 想一想，照例子完成句子。
eg: That is Danny. That is his book.
That is Danny's book.
1. This is Jenny. This is her pencil.
2. That is Li Ming. That is his glue.
3. This is Mary. This is her chair.
我国宪法规定公民负有哪些基本义务？
