下列哪一组是鸟类特有的特征（　　）

①体表有羽毛②用肺呼吸并用气囊辅助呼吸③体温恒定④通过产卵繁殖后代⑤前肢覆羽成翼⑥善于飞行．

A．①③⑥

B．②③④

C．①②⑤

D．②⑤⑥

By studying hard and _____, you will achieve your dream.        [ ]

A. give up      

B. giving up    

C. not giving up  

D. not to give up