If Barack Obama plays for the New York Yankees, then he is a professional baseball player. Barack Obama does not play for the New York Yankees. So, Barack Obama is not a professional baseball player.
A.
sound
B.
unsound
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举一反三
【单选题】处于重力场中的静止流体,z轴铅锤向上, z = 0 处力势函数 U = 0 ,且其单位质量力的分量为 f x = a x ,f y = a y ,f z = a z -g,则其力势函数为 ( ) 。