青蛙幼体生活在水中，用鳃呼吸。成体生活在陆地，进行呼吸的器官是 [ ]
A．鳃和皮肤
B．肺和皮肤
C．皮肤
D．肺和气囊
答案：B
Liu Xiang and Yao Ming are world-famous sports stars. _______ of them have set a good example to us. [ ]
A. All
B. Both
C. Neither
D. None
（）清理方法易造成钢发生氢脆。
A.火焰
B.风铲
C.酸洗
