题目

As the bell rang, the door was pushed open and _____.  A. out ran the kids                                     B. out the kids ran  C. out did the kids run                               D. out had the kids run   答案：A设正三角形ABC的边长为2，M是AB边上的中点，P是BC上任意一点，PA＋PM的最大值和最小值分别记为s和t，则s2－t2＝________．