题目

— So you haven't read the information form?— ________ makes the matter worse is that I don't have a single copy. A . It B . What C . That D . Which 答案：B4．已知PA垂直于矩形ABCD所在平面，M，N分别是AB，PC的中点．（1）MN∥平面PAD；（2）求证：MN⊥CD；（3）若平面PDC与平面ABCD成45°角，求证：MN⊥面PCD．