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