The boy decided _____ back the money to the person who lost it.A. give B. giving C. to give 答案:【答案】C【解析】句意:男孩决定把钱还给丢失的人。考查动词不定式。短语decide to do sth.:决定做某事;结合句意可选C。如图所示,已知PA⊥矩形ABCD所在平面,M、N分别是AB、PC的中点.
(1)求证:MN∥平面PAD;
(2)求证:MN⊥CD;
(3)若∠PDA=45°,求证:MN⊥平面PCD.