Sally took a photo of her friends while they               computer games.

A．play

B．are playing

C．have played

D．were playing

大多数鸟类体内具有气囊，气囊在鸟飞行时所起的作用不包括（　　）

A．辅助肺完成双重呼吸

B．减轻体重，增加浮力

C．散失热量，降低体温

D．保温保暖，使体温恒定