称四个面均为直角三角形的三棱锥为“四直角三棱锥”,若在四直角三棱锥SABC中,∠SAB=∠SAC=∠SBC=90°,则第四个面中的直角为____.
Swimming is my favorite sport.There is________like swimming as a means of keeping fit.
如图为鸟卵的结构模式图,下列说法不正确的是( )