题目

阅读理解 Tiny as they are, batshave the ability to see in the dark by using aspecial skill called echolocation (回声定位法).They make noises and wait for sound waves, or an echo, to bounceback off objects. They can tell the distance of various objects by how quicklythe sound waves bounce back off them. If no sound bounces back, they can thenfly forward. This special ability hasbeen copied in the human world for a long time, such as in submarines andplanes. But apart from helping vehicles see where they are, what ifblind people could use echolocation for themselves? American Daniel Ki如图，正三棱柱ABC-A1B1C1中，D是BC的中点，AA1=AB=1．（I）求证：A1C∥平面AB1D；（II）求二面角B-AB1-D的大小．