A:was the mountain climber rescued
B:that the mountain climber was rescued
C:when the mountain climber was rescued
D:then the mountain climber was rescued
如图所示,在直三棱柱ABC-A1B1C1中,底面是∠ABC为直角的等腰直角三角形,AC=2a,BB1=3a,D是A1C1的中点,点F在线段AA1上,当AF=____时,CF⊥平面B1DF.
传染病是由病原体引起的,病原体可以是细菌、病毒或寄生虫.(判断对错)
在Rt△ABC中,∠ACB=90°,点D在边AC上,DE⊥B于点E,连CE.