题目

Much timesitting at a desk, office workers are generally troubled by health problems. A . being spent B . having spent C . spent D . spending 答案：C8．鸟飞行时的呼吸特点是（　　）A．肺和气囊都进行气体交换B．只在肺部进行气体交换C．呼气和吸气分别在肺和气囊进行气体交换D．只有气囊在进行气体交换