3.用反证法证明真命题“四边形中至少有一个角不小于90°”时,应假设( )
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| A. | 四边形中没有一个角不小于90° | B. | 四边形中至少有两个角不小于90° | ||
| C. | 四边形中四个角都不小于90° | D. | 四边形中至多有一个角不小于90° |