题目

A. contacts     B.dependent   C . draw    D. guide    E. communication   F. challenge G. available    H.assistants    I. emergencies   J. distracted      K. victim      Human beings are so smart that we are capable of handling emergencies, reading people’s feelings and becoming close friends with each other. But are we as smart as we think?   It seems that people are gradually losing their smartness as smart phones become increasingly important ____29____ in their lives. As we rely too much on technology instead of our brains, many people have lost th如图，在四棱锥P-ABCD中，PD⊥平面ABCD，四边形ABCD是菱形，AC=6，BD=8，E是PB上任一点．（Ⅰ）求证：AC⊥DE；（Ⅱ）当E是PB的中点时，求证：PD∥平面EAC；（Ⅲ）若△AEC面积的最小值是6，求PB与平面ABCD所成的角的大小．