①Is there a map on the wall?____ A.She is a worker.
②What 's your mother 's job? ____ B.You're welcome.
③Thank you. ____ C.14.
④How many boys are there ? ____ D.She's fine.
⑤How is your mother? ____ E.Yes,there is.
____×____=____(朵)
在下列实例中,不计空气阻力,系统机械能守恒的是( )