下图所示对称结构的等代结构为( )。<img src=https://huaweicloudobs.ahjxjy.cn/04B8D7EF8BD584DEB2BB1C5EE7298379.png title=17.png alt=17.png/>
①
<img src=https://huaweicloudobs.ahjxjy.cn/50C3641AE1517140E397A4073638FBB4.png title=18.png alt=18.png/>
②
<img src=https://huaweicloudobs.ahjxjy.cn/DF4FA0C8C8E9E8DCEF557DCA4FF3C1F9.png title=19.png alt=19.png/>
③
<img src=https://huaweicloudobs.ahjxjy.cn/C9387A9BCE13D088E1E8E0F9C1BDE8BD.png title=20.png alt=20.png/>
④
<img src=https://huaweicloudobs.ahjxjy.cn/233AD2FD8CE5E7D9E9215DE6DB87A939.png title=21.png alt=21.png/>