已知函数$f\left(x\right)=\left\{\begin{array}{c}{2}^x,x\ge 0\\ \sqrt{-x},x<0\end{array}\right.$则$a=2$是$f\left(a\right)=4$成立的     （   ）

下列物种属于生物的是（　　）

①草  ②煮熟的鱼  ③珊瑚  ④钟乳石  ⑤冬眠的蛇  ⑥恐龙化石 ⑦蘑菇  ⑧家鸽．

A: Hello, is this Mr. Rogers?

B: Yes, it is.____

A: I'm from the package delivery company (快递公司); I have your package. Are you at home?
B: Unfortunately I'm at work right now. ____

A: Actually, this package needs your signature (签名). Can I bring it to your office instead?
B:____I don't have time to leave my desk.

A: I see.____

B: Yes, that would be better. Can you bring it to my home at 6 p.m.?

A: Sure, that is fine. ____

B: Great, thank you so much!