题目

  Because the shop____,all the T-shirts are sold at half price.     A. has closed down  B.closed down    C.is closing down    D.had closed down   答案：C如图，在四棱锥P-ABCD中，底面ABCD是正方形，PA⊥平面ABCD，且PA=AB=2，E、F分别为AB、PC的中点．（1）求异面直线PA与BF所成角的正切值．（2）求证：EF⊥平面PCD．