题目

_______ the little girl was hit by a truck, nobody gave a hand to the girl ______ an old lady appeared. Why were they so cold-hearted?A. When；while    B.Before; until   C. After；until    D. As； while   答案：C在四棱锥P-ABCD中，ABCD为正方形，PA⊥平面ABCD，若PA=AB，则PC与面PAB所成角的余弦值为    ．