A:should have failed
B:might have failed
C:couldn’t have failed
D:mustn’t have failed
根据欧姆定律可推导出公式R=U/I,依此可测定导体的电阻,下列说法中正确的是( )
将边长为2的正△ABC沿BC边上的高AD折成直二面角B﹣AD﹣C,则三棱锥B﹣ACD的外接球的表面积为____