So, in a quite understandable need for distraction, you're doodling and thinking more about what the plural of "series" should be than about the topic at hand: "serieses," "seriese," "seriesen," and "serii?"
因为无聊的原因， 你可能会一边在本子上乱画 一边在想，“级数”这个词的 复数形式应该是什么， 而没有想这道题本身："serieses," "seriese," "seriesen," 还是 "serii?"