设曲线$x^2-y^2=0$与抛物线$y^2=-4x$的准线围成的三角形区域（包含边界）为D，P(x,y)为D内的一个动点，则目标函数z=x-2y+5的最大值为（　　）

根据句意和首字母提示，完成句中所缺单词
①We should brush our____ (tooth) every day.
②—What’s Jane doing? —She’s b____ her sister.
③ What is she doing for v____ ?
④—How do you come to school?  —I usually ride a b____  to school.
⑤ I want to s ____ an e-mail to my aunt.

下列关于空气的说法中，错误的是（   ）