I've met her on several o____ recently.

根据短文内容，完成任务。

    Sally is an English girl. She is my English teacher, Mrs Brown's daughter. She is only five years old. She doesn't go to school. She likes playing ping-pong. Her favorite ping-pong player is Zhang Yining. Sally is in the ping-pong club. She goes to the ping-pong club every day. She plays ping-pong for two hours there. Sally has a sister. Her name is Maria. She is in our class. She likes singing. (A) She wants to join the music club, she sings every morning. (B)Her favorite singing star is Song Zuying. Maria says she wants to be a singing star like Song Zuying.

观察下面一列单项式：﹣x，2x²，﹣3x³，4x⁴，…，﹣19x¹⁹，20x²⁰，…

（1）写出第99个，第2006个单项式；

（2）写出第n个单项式．