题目

满足M｛a1，a2，a3，a4｝,则M｛a1，a2，a3｝=｛a1，a2｝的集合M的个数是(A)1                      (B)2               (C)3                             (D)4 答案：B∵M∩{a1,a2,a3}={a1,a2},∴a1∈M,a2∈M,a3M.又∵M{a1,a2,a3,a4},∴M为{a1,a2}或{a1,a2,a4},故选B.“You can do better in history，” my teacher told me. My teacher told me______ ______do better in history.