已知点F₁，F₂是双曲线$\frac{x^2}{a^2}-\frac{y^2}{b^2}=1$（a>0,b>0）的左右焦点，点P是双曲线上的一点，且$\overrightarrow{P{F}_1}·\overrightarrow{P{F}_2}=0$，则△PF₁F₂面积为 (    )

科学家最近制造出第112号新元素，其原子的质量数为277，这是迄今已知元素中最重的原子．关于该新元素的下列叙述正确的是（　　）

阅读理解

Chen Jie: Which season do you like best, Mike?

Mike: I like summer best.

Chen Jie: Why do you like summer?

Mike: Because I can eat ice-cream and swim in the lake.

Chen Jie: I don't like summer. It's too hot.

Zhang Peng: What's your favourite season?

Chen Jie: Spring. It's sunny and warm. I can plant trees and fly kites. How about you, Zhang Peng?

Zhang Peng: My favourite season is winter.

Mike: Why do you like it?

Zhang Peng: Because I can skate and make a snowman. I like snow.

已知复数z，“z+$\stackrel{-}{z}$=0”是“z为纯虚数”的（　　）