题目

 Mr. Zhang is a teacher of rich experience. He ______ English for 20 years.   A. has taught      B. will teach        C. teaches     D. taught 答案： A在北纬60°圈上有甲、乙两地，它们在纬度圈上的弧长为π2R，（R为地球半径），则甲、乙两地的球面距离为（　　）A．π3RB．π6RC．2RD．3π3R