-- ,Tommy. You can do it!
A:Come on
B:You are right
C:That's OK
D:That's all right
在数列an中,an+1=an+2+an,a1=2,a2=5,则a6的值是 ( )
书籍好比一架梯子,它引导我们登上知识的殿堂;书籍如同一把钥匙,__________。