某四面体的三视图都为直角三角形，如图所示，则该四面体的体积是（    ）

作文：
        题目：渐入佳境
       【渐入佳境】比喻兴味逐渐浓厚或境况逐渐好转。
        知识越学越有趣，乐曲越弹越流畅，风光越赏越美妙，作品越读越精彩，与人相处越来越和谐，对镜微笑越来越自信．．．．．．
        成长中，谁都有过姿态不断调整，认识不断深化，领悟逐步加深，境界逐步提升的经历。
        要求：①自选文体（诗歌除外），写一篇不少于600字的文章。②感情真挚，不得抄袭。③文中不得出现真实的人名、校名、地名。

阅读下面短文，按照句子结构的语法性和上下文连贯的要求，在空格处填入一个适当的词或使用括号中词语的正确形式填空。(每空不多于3个单词）

    Most Chinese children seem to get ____ they want. Is this a sort of spoiling(溺爱) or love? I wonder. To my surprise, it seems that the life of a Chinese child is ____ (real) hard. Without doubt , the child is the very centre of a whole circle of adults, but ____ the other hand, he or she is also ____ (expect) to start studying according to adults' wishes. Many children of my son's ____ (age) take painting classes, piano lessons, and even English lessons. It looks as if Chinese adults think ____ just playing without learning anything is ____ waste of time.

    So from this point, Western children appear spoiled, just because they are allowed____ (play). But without this sort of play, ____ can Western children develop such free and rich imaginations? In fact, the connection between this imagination and the creativity is so important in the ____ (child) future life.

若实数x,y满足$x^2+4y^2=4$，则$\frac{xy}{x+2y-2}$的最大值为（  ）