题目

单项选择：    The foreign visitors have been in Nartony for _________ three weeks.     A. more than              B. than          C. more         D. more and more 答案：A          如图，在直三棱柱ABC-A1B1C1中，∠BAC=90°，AB=AC，BC1∩B1C=E，F是AC的中点．（Ⅰ）求证：EF∥平面AB1C1；（Ⅱ）设∠B1AC1=θ，且cosθ=，试在棱AA1上找一点M，使得BM⊥平面AB1C．