某药品经过两次降价，每瓶零售价由100元降为81元．已知两次降价的百分率都为x，那么x满足的方程是（   ）

阅读下面短文，用所给动词的适当形式填空，使短文完整、通顺。每空不限一词。
     Rose is one of the funniest girls in her class. She looks like a boy, and she is good at ____ (perform). So she____ (choose) to play Harry Potter by her classmates last week. She said happily, “I'm sure that I ____ ( do )the job well because I____ (see) the movie many times already.”
    Dick, one of Rose's best classmates, is also an actor in the play. He____(like) to perform the play with Rose. He said, “We are confident that we____ (do) well in our school art festival next week.”
    Mrs. Smith, their teacher, pays much attention to their play. She often____(encourage) them. Yesterday afternoon when she ____ (watch) their performance, she took photos of them. And the photos____(send) to their headteacher next month. They hope their performance ____ (be) the best.

热带雨林是地球上功能最强大的生态系统，也是生产力最高的生物群落，但同时也是一个非常脆弱的生态系统。据此回答下列题目。