急性呼吸衰竭时，如果吸入纯氧，其时间一次不宜超过（）
A.4小时
B.5小时
C.6小时
D.7小时
E.8小时
参考答案：C
They _____ two tall buildings in the small town in the last two years.[ ]
A. built
B. had built
C. have built
D. will build
评价一个展览会网站的成功与否，就看浏览人数的多少。
