空间四个点P、A、B、C在同一球面上,PA、PB、PC两两垂直,且PA=PB=PC=a,那么这个球面的面积是____
在括号里填上合适的词。
一____火 一____波纹
一____绸子 一____树叶
一____小鱼 几____流云
—Why didn't you cry for help when you were robbed (被抢劫)?
—If I opened my mouth, they might find my four gold teeth. That would be !
眼中的晶状体相当于凸透镜,图为来自远方物体上的光经某同学眼球折光系统折射后的情况示意图。则该同学矫正视力时应配戴的眼镜是 ( )