A:其他动物的入侵
B:生活环境发生变化
C:为了扩大领地
D:为了躲避敌害
选出正确答语。
① Where is my bag?____ a. I'm from Canada.
②Is your pencil box on the chair?____ b. OK!
③What's this?____ c. It's in your desk.
④Where are you from?____ d. No, it isn't.
⑤Let's go to school.____ e. It's a toy bear.
①欲信大义于天下____
②自董卓已来____
若数列{an}的前n项和Sn=n2﹣1,则a4=( )