如图所示，如果将矩形纸沿虚线①对折后，沿虚线②剪开，剪出一个直角三角形，展开后得到一个等腰三角形．则展开后三角形的周长是（　　）

写出下列名词的复数形式。

①May I have some ____ (plum), please? Sure.

②There are seven ____ (boy) in the playground.

③The woman has two ____ (baby).

④How many ____ (peach)? There is only one.

⑤There are two ____ (library) in our school.

下列关于力学现象的解释中正确的是 （   ）