如图为某池塘生态系统中部分生物构成的食物网。

1969年，美国总统尼克松说：“十年后，到中国在核武器方面取得了重大进展的时候，我们就没有选择的余地了。”“十年后”中美之间发生的重大事件是 (    )

    Hello!My name is Bob. My classroom is not big but nice. There are 21 desks and 20 chairs. 墙上有一些画和一幅地图。Mrs Brown is our English teacher. She is from a big city.She is nice. Her husband Mr Brown is a teacher too, but he isn't in our school. Tom is my good friend. We like playing football on Saturday. Where is my football? It is under my desk.     

如图，△ABC是边长为1的等边三角形，BD为AC边上的高，将△ABC折叠，使点B与点D重合，折痕EF交BD于点D₁，再将△BEF折叠，使点B于点D₁重合，折痕GH交BD₁于点D₂，依次折叠，则BD_n= ____．