题目

 When he knew the disappointing result of the exam, he seemed quite _______. A. fed up          B. cast down    C. in vain            D. in amazement 答案：B在长方体ABCD-A1B1C1D1中，B1 C和C1D与底面A1B1C1D1所成的角分别为60°和45°，则异面直线B1C和C1D所成角的余弦值为（ ）A．B．C．D．