A:is being announced
B:has announced
C:was announcing
D:had been announced
下列哪种血管的内表面有防止血液倒流的瓣膜( )
现有一圆心角为90°,半径为12cm的扇形纸片,用它恰好围成一个圆锥的侧面(接缝忽略不计),则该圆锥的高为( )