题目

They buried _____ dead and took _____ wounded people to the hospital at once. A . a; the B . the; the C . the; a D . 不填; the 答案：B在四棱锥P-ABCD中，AB∥CD，AB⊥AD，，PA⊥平面ABCD，PA=4．（Ⅰ）设平面PAB∩平面PCD=m，求证：CD∥m；（Ⅱ）求证：BD⊥平面PAC；（Ⅲ）设点Q为线段PB上一点，且直线QC与平面PAC所成角的正弦值为，求的值．