题目

阅读表达  阅读短文，按要求完成短文后的各项任务。 Japanese children are so happy because they have three festivals each year. Including Boy’s day, they have Girl’s day and the 7-5-3 Festival. May 5th is Japanese Children’s Day. But since the festival is mainly celebrated by boys, it is usually called Boy's day. On Boy’s day, children don't have to go to school. Each family with a boy hangs up huge carp-shaped flags(鲤鱼旗). The flags are usually with three colors: black (standing for the father), red (standing for the mother), and blue (standing for the下列五个命题：（1）若直角三角形的两条边长为5和12，则第三边长是13；（2）如果a≥0，那么=a（3）若点P（a，b）在第三象限，则点P（-a，-b+1）在第一象限；（4）对角线互相垂直且相等的四边形是正方形；（5）两边及第三边上的中线对应相等的两个三角形全等．其中不正确命题的个数是（ ）A．2个B．3个C．4个D．5个